Nikon CLS Practical Guide

19. Discussion about 3 Different Flash Diffusers

2009-12-18T11:30:00.000-08:00

I decided to capture a post that I made to Nikonians.com for the Flash Forum:

The question I was answering was that the person had tried verious diffusers and was asking why he didn't see much difference between them. The bottom line is that there isn't much difference between them until you put them in specific situations.

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First, you have to understand exactly why diffusion is needed in the first place. The basic problem is that all the light from an undiffused flash comes effectively from a 'point' source. This means all the light hits the subject from one direction, and that's what makes it harsh. To soften the light hitting your subject, you have to make it hit your subject from multiple directions. This causes tiny shadows to form in multiple directions, that merge into one another adding definition to facial features giving your subject a more natural look.

To make the light from the flash hit your subject from multiple directions, you have to use a diffuser, and the bigger the diffuser, the more paths the light will follow while traveling to your subject, and the softer it will be.

When you are indoors and have a white ceiling (that is not too high), and white walls, you can make your flash really soft if you send the light in all directions, so it bounces off every wall and ceiling before arriving at the subject. Then, you get amazingly soft light.

So, any diffuser that will send the light in all directions indoors in a room with white walls and ceiling, will make equally soft light. The small snap-on diffuser that comes with the SB-800 will do a remarkable job of this if you simply point it straight up.

However, there is more to this. In addition to making your light soft, you normally want to send a bit of light directly toward the subject to make catchlights in the eyes and lift shadows on the face. This is called 'direct' light, and if the direct light is coming from a tiny source, then you are adding harsh light into the image, which cancels out some of the softening you have created with the bounce.

So, the overall quality of the light on your subject depends on controlling the softness of both the direct light and the bounced light and the ratio between them.

So, let's look at the various diffusers you mentioned. If a bounce card is used without any other diffusion, you may already see that the direct light will be coming from a source that is the size of the bounce card. If this card is small, then the direct light will be harsh. This is what happens with the pull-out bounce card in the SB-800. It makes very harsh direct light, so it provides no diffusion at all by itself. Only the light that gets around it and is allowed to bounce off walls and ceiling will be soft. You can make it do a good job by pointing it up and adjusting how much of the bounce card is sticking out.

With the Demb flash diffuser pro, it uses a larger bounce card which softens the light more than the small card in the SB-800. You can compare softness of bounce cards by comparing the area of the bounce cards. The SB-800 card is 1.4 x 1.75 or about 2.5 sq inches. I don't own a Demb Flash Diffuser Pro (DFDP), but from the pictures it looks like its bounce card is about 3 x 3 inches or 9 sq inches. This will provide slightly more softening of the direct light than the smaller SB-800 card. Then, the DFDP adds a 'Front Diffusion panel'. This adds about 2 sq inches to the overall size of the direct light source and will cause part of the light to scatter, assuming an inverted funnel shape towards the subject. By itself, this diffusion panel will not add any softening, because the active area is about the same size as a flash. However, if you are indoors, some of the scattered light may hit the walls and arrive back at the subject. This light will be soft, but much of it will hit the subject from the sides, so it won't soften the face that much. The DFDP is a very versatile system that can produce great results once you fully understand how to use it.

The Lumiquest Mini Softbox softens only the direct light. It produces no bounce, so the area of the active surface of the box can be used to compare it to bounce cards. From the website, it says it will fold flat to 3.25 x 4.5 inches. This is about the same size as the active surface of the box so the area is about 15 sq inches. Therefore this will provide more softening of the direct light than the DFDP.

Then there is the Light Sphere II (LS) which comes with its 'inverted dome'. With the dome installed, and with it pointed straight up, it will cause the flash to scatter in all directions. Light will then bounce off the ceiling and every wall in the room, just exactly like the small snap-on diffuser that comes with the SB-800. This makes the effective light source the size of the entire room. This could be a total effective area of 500 sq feet. Compare that size to any bounce card there is and you see that there is no comparison. You get extremely extremely soft light, probably only to be outdone by an overcast day, when the entire sky becomes the diffuser size.

However, the advantage of the LS over the small snap-on diffuser, is that its active cross-sectional size is about 3 x 4 inches or 12 sq inches. This means that its direct light will be much softer. The direct light from the LS is not as soft as the Lumiquest Mini Soft Box and about the same as the DFDP.

The real power of the LS is that when you have a good ceiling bounce, you can remove the inverted dome and take full advantage of it. This is when you get maximum softening. Not only do you get a maximum bounce off the ceiling, but you still get a bounce off the walls and nice soft direct light as well. You also have the option of tilting the LS backwards to reduce the direct light while still retaining maximum room bounce. This is why the LS works so well.

Lastly, I should point out that as you move farther from your subject, the apparent size of the source grows smaller and smaller. This makes the direct light get harsher and harsher. Once you are beyond about 15 feet, none of the diffusers will have much of a softening effect on the direct light. When you are 15 feet or more away, you have to rely entirely on the room bounce for softening. Then, if you are outdoors (and within 15 feet), where there is no bounce, you have to rely entirely on the softness of the direct light. This is when the Lumiquest Mini Softbox beats the DFDP and the LS.

18. Camera & Flash Cookbook for Any Lighting Situation

2009-12-05T07:36:00.001-08:00

I am often asked how to set up your camera and hot shoe flash for a given lighting situation. This post will give you a 'cookbook' approach that should lead excellent results.

NOTE: For this post I am assuming regular flash sync - Not Auto FP High Speed Sync

NOTE: On some speedlights, including the SB400, SB700, and the Pop-Up flash, there is no selector for iTTL and iTTL-BL. To switch between these two modes on these speedlights, you switch the camera metering mode. Matrix and Center-Weighted force the speedlight into iTTL-BL mode. Spot metering forces the speedlight into regular iTTL mode.

DETERMINE THE AMBIENT LIGHTING CONDITIONS:

Use your camera to measure the light! Here are steps:

1) Camera in Manual mode
2) Flash turned OFF
3) Fixed ISO 400 (not Auto ISO)
4) Aperture: f/4.0
5) Aim your camera at the area you want to measure
6) Adjust the shutter to zero the meter

The resulting Shutter speed then will indicate the ambient lighting condition you are in as follows:

1) Low Ambient: Shutter 1/30th or less
2) Medium Ambient: Shutter 1/30th to 1/250th
3) High Ambient: Shutter above 1/250th

CAMERA AND FLASH SETTINGS FOR EACH LIGHTING CONDITION

LOW AMBIENT:In low ambient conditions, your flash will be primary and essentially the only light on the subject. The ambient will contribute only to the background exposure.

Recommended Initial Settings: Camera Manual, Flash iTTL, ISO 400, f/4.0, 1/80th shutter.

The flash system will control the exposure of the subject no matter you do to the ISO, Aperture, or Shutter (within the range limts of the flash).

The shutter will primarilly control the background exposure. Increase the shutter to stop ghosting at the expense of a darker background. Decrease the shutter to brighten the background, at an increased risk of ghosting.

The aperture will primarilly control depth of field. Widen the aperture to decrease depth of field, increase background exposure, and increase flash range. Narrow the aperture to increase depth of field, decrease background brightness, and reduce flash range.

HIGH AMBIENT:
In high ambient conditions your flash will be adding FILL. This means the flash will be secondary to the ambient light in creating your images. The flash will brighten the shadows on the subject's face and clothing. The power of the flash must be adjusted to balance with the ambient light to make the subject equal brightness to the background ambient. You should use iTTL-BL flash mode to allow the flash to adjust itself automatically to balance the subject with the ambient.

If direct sunlight is hitting the subject's face, move the subject into the shade. If that is not possible, turn off the flash to avoid overexposure (blow out) of the subject's face.

Recommended intitial settings:
Camera P mode, ISO 200, iTTL-BL
This is the simplest setting I recommend at first until you more fully understand everything. These settings will add nice Fill to your shots.

MEDIUM AMBIENT:
Medium ambient is by far the most difficult situation to use your flash. It is also the time when if you use your flash right, it will greatly improve the quality of your images, but if you use it wrong, it will destroy your images.

In medium ambient you have to make several more decisions to determine the initial settings. You have to decide how you want to shoot:
Do you want the flash to be primary?
CAN you even make the flash primary?
Or do you want the flash and ambient to balance on the subject?
Or do you want the flash to be only light Fill and the ambient primary?
Or do you have to turn off the flash and shoot available light?

Then you might ask, why would I want the flash to be primary?
Well, the most frequent reason would be when the ambient light was some funny color, like bright mercury vapor lamps (gymnasium). The flash is the color of daylight, so you would then have a weird multicolored image since there is no single white balance that can be chosen. The only way to get proper colors in this situation is to either turn off the flash and use a white balance that matches the ambient or eliminate the ambient by making the flash primary.

A second reason you might want the flash to be primary would be if you are shooting a subject that is moving quickly (dancers, runners). You can eliminate ghosting and blur if you make the flash primary, since the flash will then freeze the motion because its duration is only about 1/1000th sec.

HOW TO MAKE THE FLASH PRIMARY:

To make the flash primary, you have to make the flash overpower the ambient light. This must be done with the flash in iTTL mode by decreasing the exposure by three stops or more with shutter and ISO from the ambient setting you measured with the camera meter above.

Example 1: Assume the results of your metering step above came out to f/4.0, ISO 400, and 1/60th. You would then recognize that you were in Medium Ambient Conditions, so you must immediately change the ISO to 200. This decreases the ambient exposure one stop.

Then increase the shutter by two stops to 1/250th (double it twice and pick the closest setting - 1/60th-->1/120th-->1/240th). Notice that the highest shutter speed you can select when in regular flash sync (on a D200) is 1/250th. Now, you have changed the settings so the camera will underexpose the ambient by three stops. This example is right on the edge of where you might have to turn off the flash and shoot available light or switch to TTL-BL and go for balanced fill. In fact, if your camera is a D80 it has a maximum flash sync speed of 1/200th, so you would not be able to fully overpower the ambient in this situation.

IMPORTANT POINTS:

1) It is important to understand that when using the flash in regular iTTL mode, the flash metering system does not measure the ambient, so whatever flash power it determines is as if you were in pitch dark conditions. This causes the ambient to add to the flash on the subject and if the ambient is not totally overpowered, it can easily overexpose the subject. So, if you cannot totally overpower the ambient, and you still want to use iTTL mode, YOU MUST REDUCE THE FLASH POWER using the FEC button to avoid overexposure. The amount you must reduce it is a judgement call, making flash in medium ambient conditions that much more difficult.

2) In iTTL-BL mode the flash DOES measure the ambient, and then sets the flash to make the subject equal brightness to the ambient. This will only work well, however, if the ambient light is the same color as the flash (ie, daylight). If the ambient is something light mercury vapor, don't use iTTL-BL. Also, it is important to understand that TTL-BL will only work correctly when the camera is set for a normal ambient exposure. P mode makes this easy, since the camera will automatically set itself for a normal ambient exposure.

Note: On the SB-600, 800, and 900 Speedlights there is a mode selector to put the flash in either iTTL or iTTL-BL modes. ON the pop-up flash and the SB-400 and SB-700 speedlights, you control the selection of iTTL or iTTL-BL from the camera using the metering mode selection. If you select Matrix or Center-Weighted metering, you will get iTTL-BL mode. If you select Spot metering, you will get iTTL mode. It shows this in the display of the SB-700 but not the SB-400.

Example 2: Assume the results of your metering were ISO 400, 1/200th, and f/4.0. You should immediately see that you will have major problems reducing the exposure of the ambient by three stops. You can't double the shutter, because you are already near maximum flash sync speed. That leaves only ISO and Aperture. You can reduce the ISO to 200 and that gets you one stop, but the only way to get the other two stops is to stop down the aperture by two stops (f/4.0 --> f/5.6 -- f/8). But if you change the aperture to f/8, the flash range will become less than six feet or so (using a diffuser), so that may not be an option. In fact, if this is your situation, you are forced to abandon the idea of overpowering the ambient light.

You must now consider balancing the flash to the ambient.

HOW TO BALANCE THE FLASH TO MEDIUM AMBIENT CONDITIONS

If you find yourself in a situation like Example 2) above, you could decide to turn off the flash and shoot available light or you could decide to balance the flash to the medium ambient.

There are two approaches to balancing the flash to medium ambient conditions:

1) In iTTL mode you reduce the flash compensation manually by trial and error until the right flash power is determined. I am not going to discuss this method.

2) In iTTL-BL mode, the flash system will set itself automatically. This is the approach I will now discuss.

Recommended Initial Settings for allowing iTTL-BL to balance automatically with medium ambient:

Camera P mode, Flash iTTL-BL, ISO 200. (notice that these are the same settings as for High Ambient conditions).

This will give you nicely balanced images with bright backgrounds and bright subjects. You might want to apply some negative flash compensation, say -0.3 ev, to keep it from looking like the subject is jumping out of the picture.

You might be tempted to use camera A mode, and it can be done, but you have to be very careful to prevent overexposure. The shutter is limited to 1/250th (or 1/200th) flash sync speed, and since it can't go any higher than that, if you choose an aperture that is too wide, the shutter will bang into the limit and your images will all be overexposed. P mode fixes this problem.

Incidentally, among professional photographers the joke is that P mode stands for 'Professional' mode, because we all use it in bright ambient when shooting fill flash. For some reason, beginning photographers avoid P mode like the plague. P mode has its important uses, and shooting iTTL-BL is one of them (especially in bright ambient).

CONCLUSIONS:

1) You must learn to use your camera metering to measure the ambient light

2) Low ambient situations are the easiest to learn to shoot flash. These are also the most common, and occur almost all the time indoors at parties, receptions, family time, etc. Use the recommended settings for exceptional flash shots every time.

3) High ambient light situations are also easy to shoot flash. This situation occurs every day outdoors during daylight. Let the wonderful iTTL-BL mode do all the work for you.

4) Medium ambient light makes flash photography very complicated, but the results are well worth it. This is the situation you have to really study in order to learn how to do it right.

17. Why use Camera Manual when Using Flash Indoors?

2009-11-09T11:33:00.000-08:00

I am often asked why you should use your camera in Manual mode when shooting flash in low ambient conditions, and why is that better than shooting in Aperture priority. This study will explain the reasons, plus I will show the effects of Rear Sync versus Front Sync when there is any subject motion involved in flash pictures.

But first it is important to fully understand that when you use your camera in Manual mode and your flash in iTTL, the flash handles the exposure of the subject, as long as the ambient is dim, and the subject is in the center of the frame. So the image is still exposed automatically even though the camera is in Manual mode.

Here are my conditions that do not change during this study:
- Dim ambient light
- ISO 400
- f/2.8

Image 1: Test Image, Flash off, 1/60th, Manual Mode. The purpose of this image is to indicate how much ambient light there is. You can see that at 1/60th, it is about two stops underexposed. When using the camera in Manual mode and the flash in iTTL mode, you want the camera controls to always underexpose the ambient by about two to three stops. Also, I moved my hand, so you can see what motion blur looks like without the flash.

Image 2: Flash ON, Aperture Priority. I was moving my hand toward my face. Look closely at my fingertips. That orange blur on each finger is called ghosting or ghosting trails. The flash froze my moving hand and then the ambient caused the ghosting. Also, noticed the direction of the ghosting trails. The ghosting trails appear to preceed the motion of my hand. This is what we call ghosting trails on the 'wrong' side of the motion. The position of the ghosting trails was caused by using Front Sync.

Image 3: Flash ON, Camera Manual Mode, 1/120th shutter. I was moving my hand toward my face. Notice that there are no ghost trails. This is because the shutter speed was increased high enough to eliminate the ambient from the image.
Image 4: Flash ON, Camera Manual mode, Rear Sync, 1/60th shutter. Again, I moved my hand toward my face. Look closely at my fingertips. Now, you can see ghost trails going in the opposite direction. This is because Rear Sync was used, which caused the flash to fire at the end of the shutter cycle rather than the beginning of the shutter cycle as in the previous images. Now, the ghost trails are on the 'correct' side of the motion.
Image 5: Flash ON, Aperture Priority mode, Rear Sync, 1/15th. Again, I moved my hand towards my face. Notice the much greater amount of ghosting. This is because when you select Rear Sync in Aperture Priority, you also get Slow Sync, and Slow Sync, causes the shutter speed to reduce to fully expose the ambient. This also caused overexposure.

Image 6: This has all the same settings as Image 5. I just wanted to show the great sensitivity to movement. All I did was move my head.
Conclusions:

1) When shooting flash in low ambient conditions, like normal indoor artificial lighting, it is best to use Camera Manual mode with Rear Sync to control ghosting.

2) Using Aperture priority with Rear Sync is not a good choice for dim ambient conditions. It will lead to overexposure and way too much ghosting and general motion blur.

3) Using Aperture Priority is also not a good choice when using Front Sync when shooting in dim ambient conditions, because the camera will always select 1/60th sec shutter (Flash Shutter Speed). This is too slow to stop ghosting during motion like a speaker waving his arms or a couple dancing.

Recommendation:

Always use Camera Manual Mode with Rear Sync when shooting flash in dim ambient conditions so you can control the ghosting.

Never use Aperture Priority when shooting flash in dim ambient conditions, because you cannot control the ghosting.

16. Does moving the focus points affect flash power?

2009-10-21T06:12:00.000-07:00

Lately there have been several comments made by experienced photographers stating that selecting different focus areas in the camera will change the place in the image that the flash meters from. I knew that this wasn't true based on my knowledge gained by designing flash and camera integrated circuits. I knew that the flash meters the entire frame based on center weighted brightness. However, I needed some proof, so I took the following series of images.

Firts, I arranged a white panel and a black panel so that they split the frame exactly in two. These images are unedited, directly out of my D3, low ambient conditions, f/5.6, 1/200th, ISO 400, regular TTL.
Image 1) In this image, the focus point was moved to the far left in the center of the white panel (the panel appears gray, because the flash system has averaged the center weighted frame and chosen a power that balances the entire image to 18% gray.

Image 2) In this image, I moved the focus spot all the way to the right side in the middle of the black panel. Notice that this image is exactly the same exposure as Image 1) showing that the position of the focus point is not used for the regular TTL metering system.

In the next few images, I investigate the effects of the focus system on TTL compared to TTL-BL.

Image 3) In this image the flash was in TTL and the subject was placed in the left part of the frame and the leftmost focus point selected. You can see that the statue is slightly overexposed from being off-center, but that is not what I am studying this time.

Image 4) In the next image, (still in TTL mode) I moved the focus point to the far right on the rope extending below the hats, and allowed the camera to focus there. You can see that moving the focus point did not affect the flash metering. The subject (although now out of focus) is exactly the same brightness as the previous image, as is the background.

Image 5) In the next image I switched to TTL-BL mode and placed the focus point back on the statue. You can see that the statue is no longer overexposed, as the balancing equations try to balance it with the darker background.

Image 6) In the next image I remained in TTL-BL mode and moved the focus point to the far right so the rope below the hats would be in focus. Immediately you see a change in the flash power. The statue is now overexposed and the background is much brighter. This is what has been causing even experienced photographers to think that the focus selection has an effect on flash power. However, the change is not being caused by moving the focus points. It is being caused by the change in focus distance. In this image the hats are about 15 feet behind the statue, and in TTL-BL mode, the distance is the prime factor that is used to set flash power. So, the farther distance caused the power to increase.

Image 7) In the next image I left the flash in TTL-BL mode, and switched off the auto focus and moved the focus point back to the statue. the important thing to notice is that the exposure of this image is identical to the previous image. This proves that moving the focus point does not affect flash power.

Image 8) In this image I moved the focus point back to the hats. Again the exposure did not change.

Image 9) In this image, I moved the statue into the center of the frame and selected regular TTL. Now the exposure of the statue is correct (the color is now accurate as well). This shows that even with all of the advancements of how TTL-BL deals with a dark background, regular TTL is still better as long as the subject is centered properly in the frame and occupies about 20% of the frame so the monitor preflashes can meter it accurately.

CONCLUSION:

These images prove that moving the focus point does not affect flash metering in either TTL or TTL-BL flash modes. The change in exposure that has been observed in TTL-BL mode when moving focus points is due entirely to the change in focus distance as reported by the D lens.

15. When is Full Power Flash not Full Power?

2009-03-20T05:04:00.000-07:00

When you set your flash in Manual mode and full power, you expect to get a full power flash, right?

Well, you would be wrong! At least part of the time.

Many of you will be surprised to learn that even with your flash in Manual Mode you will only get full power when the shutter speed is slower than about 1/125th sec!

This is because when the flash is in Manual mode and mounted on the hot shoe, it turns ON and OFF with the Sync pulse that fires it through the center terminal.

When the flash truly fires at full power, like when you hold it in your hand, it completely discharges the capacitor, and it takes longer to do this complete discharge than the time available at the flash sync speed of 4 ms (1/250th sec).

In fact, the tail of the flash pulse goes out to about 6 - 8 ms before it is completely gone. This portion of the tail is well below the half power point that is used in the specification to define the length of the full power flash. This is how they get by with saying the flash is 1/1000th sec when at full power. That's the length of time between the half-power points on the waveform.

So, when the flash is in the hot shoe, the flash fires when the Sync signal is applied as the shutter opens, and it is quenched when the Sync signal goes away as the shutter closes. This stops the complete discharge of the flash if there is any energy left in the capacitor.

Now, when the shutter is at 1/60th (17 ms), the flash has plenty of time to completely dump all the charge on its capacitor. However, at faster shutter speeds, like 1/250th (4 ms), there is still significant charge left in the capacitor when the flash is squelched.

This remaining charge is also why the flash will recycle faster when at 1/250th shutter than when at 1/60th, even when the flash is in Manual mode and set to 1/1 (full power).

This also explains why the image from the 1/60th shutter is brighter than the image from the 1/250th shutter when the flash is fired at full power. The tail that is cut off at the higher shutter speeds but present in slower shutter speeds, contributes to the exposure.

14. Basics of Flash Brackets and Diffusers

2008-08-12T05:23:00.000-07:00

I am often asked about diffusers and brackets. Photographers mostly want to know what they do and which ones are best.

Well, both the bracket and the diffuser were introduced into photography to solve the problems of the small flash.

The problems of a small flash, when used pointed directly at the subject, are that it makes harsh, flat light, that often causes 'Red-Eye', and, when turned to the vertical, casts a harsh shadow to the side of the subject.

HARSH LIGHT

In general, harsh light comes from a physically small 'apparent' source. An infinitely small source is called a 'point' source, because the light comes from a single point.

I use the word 'apparent' because the both the size of the source and its distance from the subject cause the apparent size to change from the perspective of the subject. For example, one of the harshest light sources is also one of the largest - the sun. It is huge - over 800,000 miles in diameter, but it is so far away 93,000,000 miles, that it acts as a point source to a subject here on earth. In other words, its apparent size is very tiny. This type of light casts very harsh shadows and makes facial features appear dull and lifeless.

The solution to harsh light is diffusion. Diffusion is the process of scattering the light from a point source so that it acts like a much bigger source. Think of the sun on an overcast day. The light still comes from the sun (effectively a point source), but the clouds scatter the light as it falls on the earth so that is becomes some of the softest light you can find. In effect, the clouds have changed the apparent size of the sun from a point source to a source the size of the entire sky. You can make extremely good portraits on an overcast day outdoors, where the facial features have nice texture and seem to come alive.

The shoe-mounted flash is also a very small physical size size (about 3 square inches) and is an effective point source beyond a foot or so. Consequently it makes extremely harsh light that is not suitable for portraits without modification (diffusion).

Now, some people think that simply putting a diffuser in front of the light will make it soft, but this is not the case, because that does nothing to increase the apparent size of the flash.

There are two ways to create diffusion when using a small flash. 1) utilize a physically large 'retransmission' system or 2) bounce off large surfaces. You can also use a combination of these two.

A 'retransmission' system is usually made from a large piece of translucent material that is lit by the flash that then retransmits the light evenly across its surface. The Gary Fong Light Sphere II is such a system. It looks like an inverted Tupperware bowl that is mounted on the flash. Then, the flash is pointed straight up and when it fires the whole translucent bowl lights up. In fact the cross section area of the bowl is about 16 square inches, or about five times larger than the flash itself. This softens the light so that portraits made within about five feet are noticeably softer than the bare flash.

Another even better 'retransmission' system is a soft box. If you get one that is 36 inches by 48 inches, that's 1728 square inches of area. This is 108 times larger than the Light Sphere, and creates light that just as soft as the light on an overcast day outside.

The problem is portability. How big is too big to carry mounted on your flash. My personal feeling is the the Light Sphere II is about as large as you can conveniently handle under normal circumstances.

As you probably know, the SB-800 flash comes with a small snap-on diffuser. If you own an SB-600, you can buy an after-market snap-on diffuser from companies like Sto-Fen. Just looking at this snap-on diffuser, you can see that it is physically very small and the question is: does it work? It is obviously not much bigger than the bare flash itself, so it cannot increase the apparent size of the flash directly. However, think of this small diffuser attached to the flash and the flash pointed straight up. Then when it flashes, the light scatters to the sides, up, back, and everywhere but down. This means some of it goes directly to the subject, but most of it goes towards walls and ceiling where it can bounce, sending some of the bounced light back at the subject. The key is that the bounced light hits the subject at a different angle than the direct light. This different angle is what makes the apparent size of the flash much larger. So, there is a small portion of the light which is still harsh that goes directly at the subject and the rest that bounces off walls and ceilings is soft. The small amount of harsh direct light is what makes the 'catch-light' reflection in the eye of the subject and is not objectionable.

The reason the Light Sphere II works so well is that it is large AND it scatters light in all directions. This makes it work extremely well indoors where the direct light is much softer than the small snap-on diffuser, and the walls and ceilings all increase the softening effect more than with the small snap-on diffuser.

Now, outdoors, all the light that scatters from a diffuser is lost forever. It never makes it back to the subject. This leaves the subject lighted entirely by direct light from the flash which is still harsh because the apparent size of the flash is still small. So, outdoors, the only thing that happens if you use a diffuser is that your flash batteries run down quicker, because the flash has to flash at a much higher power to light the subject than normal. The bottom line is that outdoors, never use a small diffuser; just use the flash direct. However, the Light Sphere II is useful outdoors due to its large size which softens the direct light out to about five feet.

SHADOWS AND RED-EYE CONTROL

Flash shadows are always objectionable and you should always strive to eliminate them.

Red-eye is also extremely objectionable. It is caused by the flash reflecting off the retina of the eye and back into the picture. Red-eye is best addressed while taking the picture, but can also be somewhat corrected during post-processing. I will discuss how to avoid causing it in the first place.

The solution to the red-eye and shadow problem is to raise the flash higher above the camera while keeping it directly above the lens axis. This can be achieved with either a flash bracket or a flash diffuser that sits high above the flash.

The flash bracket attaches to the tripod mounting hole on the bottom of the camera and the flash mounts on its top directly above the lens. It raises the flash several inches higher than when the flash is in the hot shoe. Then, there is a cable that attaches the hot shoe on the bracket to the hot shoe on the camera, so that the camera thinks the flash is still in the hot shoe.

The main reason for raising the flash higher above the camera is to force the flash shadow down behind the subject where it can't be seen. The LS-II diffuser also raises the flash high enough to fix the shadow problem when the camera is in the horizontal orientation.

Also, by raising the flash higher above the camera, the angle of the direct light causes the red-eye reflection to fall below the camera lens. This simple step elimintes 99% of the red-eye problems. It is important to note, however, that the farther you are from the subject, the higher the flash needs to be raised to avoid red-eye. For instance, if you are using a telephoto lens, and you zoom tightly onto a person's face from 30 or 40 feet away, you risk red-eye.

So, the solution to red-eye is also the bracket, although with a DSLR, the flash is already much higher above the camera than with a point-and-shoot, so red-eye problems are less of a problem.

There is another very important feature of a flash bracket - it fixes the dreaded 'side shadow'.

When you turn the camera to portrait orientation (vertical), and the flash is mounted in the hot shoe, you cast a shadow to the right of the subject on a wall. This shadow is very unprofessional and is to be avoided at all cost. With a flash bracket either the camera or the flash rotates so that the flash always remains directly above the camera. By locating the flash above the camera, the shadow will now fall behind the subject and out of sight.

This is a disadvantage of using the Light Sphere II diffuser. The LS-II still casts the side shadow, although it is less defined as with a bare flash. Often when I want vertical orientation while I am using the LS-II, I simply shoot horizontal and crop to vertical later in post processing.

Another important consideration with a flash bracket is the direction you rotate the camera when going to to vertical. Most professional photographers rotate the camera counterclockwise so that the alternate shutter release button is on top and useable. But some flash brackets for some unexplicable reason force you to rotate the camera clockwise, so you have to be careful when buying. I use a Custom Brackets 'CB Junior' with my D200 and SB-800. It is relatively inexpensive at about $110 and works well.

13. SB800 A and AA Modes

2008-07-22T18:15:00.000-07:00

So, what are the A and AA modes are on the SB800?

Well, first, there are actually three non-TTL modes that use this technology: 1) A mode flash in hot shoe, 2) AA Mode, flash in hot shoe, 3) Commander AA Mode, wireless flash in Remote Mode.

These are all 'automatic' modes on the flash that do not use TTL (Through The Lens) technology. Instead, these modes use a small clear window in the flash itself to determine when the reflected light from the subject is correct. Take a look at your SB800 from the front. You will see the small round clear window on the lower left side pointed straight forward.

A Mode, Flash in Hot Shoe:

The A Mode with the flash in the hot shoe is the oldest and simplest technology used for non-TTL automatic flash control. I first used an 'Automatic' Vivitar flash in the early 70's, and it worked extremely well. This technology still works extremely well today in the SB800.

The SB800 flash defaults to AA mode, which I will discuss in a moment, so to put the SB800 flash in A mode you have to go into the menu on the flash and choose A mode. Press ON/OFF to exit leaving the flash in A mode.

The concept of the A Mode is very simple. The main flash pulse fires (turns ON) when the shutter is pushed, and there is a thyrister circuit inside the flash that measures the reflected light from the the subject through the little clear window. The flash pulse stays ON until that reflected light accumulates to a certain amount, and then the circuit turns the flash OFF.

Of course the flash needs additional information about the camera settings in order to work. In the older Automatic flashes, you had to enter the ISO and f/ stop manually into the flash. With the SB800 in A Mode, the ISO is picked up automatically from the camera through the hot shoe, but you still have to enter the f/ stop manually using the rocker switch on the back of the flash.

Then you simply aim and shoot. The flash takes care of the exposure automatically.

One very nice thing about the A Mode on the SB800 is that there is no preflash pulse sequence to cause people or especially animals to blink.

NOTE: If you want to observe the preflash pulse sequence separately from the main flash pulse, simply put your camera in flash Rear Sync, and set a long shutter, say 1.6 sec, and push the shutter. The preflash sequence will occur at the instant the shutter is pushed, and the main flash pulse will occur at the end of the shutter sequence right before the curtain closes. This is handy for verifying what I am explaining about no preflash pulses when using A Mode.

AA Mode, Flash in Hot Shoe:

The AA Mode is the default mode for the 'Automatic' non-TTL flash modes. If your flash is sitting in the hot shoe, and you cycle through all the modes, you will see AA in the list unless you have changed it to A.

The AA mode is similar to A mode except for two things: 1) the flash automatically picks up both the aperture and the ISO that you have set on the camera, so AA mode is really fully automatic, and 2) a preflash pulse sequence is used.

To use AA mode, simply select it on the flash, point and shoot. The flash measures the amount of reflected energy from the Preflash through the small clear thyrister window. In this mode the flash does not used the main pulse to set the power.

The preflash is also used by the camera in AA mode to calculate the white balance.

Another useful byproduct of the fact that the AA flash uses a preflash is that you can use the Flash Value Lock (FV Lock) function. This helps with the fast blinkers when taking group shots.

In addition, there is another way use AA mode that eliminates the preflash sequence. You simply put the flash in SU-4 Mode, while in the hot shoe, and when AA mode is selected, it works by measuring the main flash through the thryrister window just like the A mode does.

Commander AA Mode, Flash in Remote:

The third 'automatic' flash mode is the Wireless AA Mode. The flash is set to Remote Mode and placed separate from the camera with the small round red IR window facing the commander. Then AA Mode is selected on the Commander menu on the camera. In this mode, the Commander sends a preflash pulse to the flash telling it to use its internal AA mode followed by the command to fire the main pulse. The remote flash also fires a preflash pulse which is used for setting its own power via the thyrister 'eye' as previously discussed.

So, When would you use these modes?

Flash A mode or SU-4 AA mode are useful when shooting pets. These modes eliminate the problem of pets who can close their eyes so quickly that they are closed by the time the main flash fires. The alternatve is to use TTL mode and FV Lock. I use both techniques. Some animals are greatly bothered by the flash, so I use A or SU-4 AA mode just to reduce the flashes the animal has to put up with.

It turns out that Flash A and SU-4 AA modes measure the reflected energy quite evenly across the frame, so it is useful for group shots as well.

Flash A and SU-4 AA modes do a very good job of setting the flash to the correct power most of the time, but these modes still suffer from the same problems as TTL, ie the flash power is set too high when the subject is black (or dark colored) and too low when the subject is white (or light colored).

One 'gotcha' to remember for both AA and A mode, especially if you use them in an umbrella, is that you must always turn the body so the thyrister 'Eye' is pointed at the subject.

12 Speedlight Trick #1

2008-04-01T08:41:00.000-07:00

One of the situations I run into regularly is that I am fairly far from a subject who is in a dim ambient setting, and there is something closer to me that is affecting the flash metering. This could be a plant, a wall, another person, a wedding cake, or anything that can reflect the preflash pulses back to the camera.

For example, lets say your subject is standing inside a dim room and you want to shoot a picture of her from outside the room through a doorway. If you simply set TTL on the flash and place the subject in the center of the frame and shoot, the reflections from the doorway will make the system think your goal is to shoot the doorway and you will get a great shot of a slightly overexposed doorframe and the subject will be pitch dark. The doorframe will be slightly overexposed, because it is not in the center of the frame, but it will still cause the flash to reduce power drastically leaving the subject dark.

The trick is to first aim the head of the flash forward and remove any diffusers that don't push the little switch on the flash head (so distance information can be used by the flash). Then switch the flash to TTL-BL and frame the shot and hit FV Lock.

Then, the distance to the subject is used in addition to measuring the strength of the reflected preflash pulses from the center of the frame. The flash sees that the distance is much farther away than the doorframe, so it increases its power accordingly. This usually doesn't brighten the subject fully, but it definitely brightens the subject far more than using TTL. Of course, once the subject is brightened to proper exposure, the doorframe will likely be blown out, but that can't be avoided.

The way to make this trick easy to implement is to start with the camera in manual mode and matrix metering and the flash in TTL-BL. Then, switch the camera to spot metering. That will force the flash automatically to TTL which is usually best indoors in dim ambient. Spot metering doesn't do anything to the camera settings since manual mode has been selected. It only affects the flash mode.

Then, when you run across the situation mentioned above, switch the camera metering to matrix, and the flash will automatically switch back to TTL-BL, and the shot will be exposed the best way possible. However, the flash may still need increased compensation to get the exposure on the subject exactly right.

If you don't want to remove the diffuser, another way to do this is to zoom in on the subject and hit FV Lock. The flash metering system will only look at what is in the frame center weighted. Then, zoom back out to include the doorway or whatever is between you and the subject, and take the shot. Of course, the doorway will be blown out, but that's normally OK in this type of shot.

11. Other Great Flash Sites

2008-03-27T13:55:00.000-07:00

There are lot's of other great blogs that discuss flash techniques, so I decided to reference them here. The things I put in my blogs are extensions and amplifications to things I have read in these other sites plus additional things I have discovered during my wedding and event photography.

Planet Neil
Neil van Niekerk is a wedding photographer who uses both Nikon and Canon speedlights and describes his methods for making the lighting from the flash subtle and with a natural appearance.

Strobist
David Hobby describes the equipment needed and dozens of ways to use speedlights off-camera. I highly recommend you read his blogs under 'Lighting 101'.

10. Auto FP High Speed Sync Explained

2008-03-14T13:54:00.001-07:00

One of the most confusing aspects of the Nikon flash system is this thing called 'Auto FP High Speed Sync'. This blog will hopefully clear up the mystery about this mode.

Before you can understand FP High Speed Sync, you have to understand what FP means and how the Normal Flash Sync works.

FP stands for Focal Plane and it refers to the type of shutter used in most modern DSLR cameras. A focal plane shutter is actually two precisely timed curtains positioned between the lens and the sensor that can either block light from hitting the sensor or allow light to hit the sensor. The reason there are two shutter curtains is to be able to get much higher effective shutter speeds.

It is important to understand is that these curtains open and close in exactly the same amount of time. So the the shutter speed is set by timing between the start of the first curtain opening and the start of the second curtain closing.

Notice that the entire sensor will be open to the light at every shutter speed up to the speed of the curtain movement itself. This is the maximum normal Flash Sync Speed. To say this another way; at all speeds up to the maximum normal Flash Sync Speed, the first curtain completely opens before the second curtain begins to close. At any shutter speed higher than this, the second curtain will begin closing before the first curtain gets fully open, thus never exposing the entire sensor at any one time. At really high shutter speeds, this results in very narrow 'slit' of light that travels across the sensor.

These two curtains travel vertically across the opening from the top to the bottom, and this in itself causes some strange effects when using high shutter speeds (small slit). If you shoot something that moves horizontally really fast like a race car from the side, you can sometimes see that the wheels seem to lean forward a bit because the top part of the wheel was exposed after the bottom part and the top moved forward a little bit as the slit moved from bottom to top. Of course, the image is inverted on the focal plane, exposing from the bottom to the top of the image, and that's why race car wheels lean forward even though the shutter that moves from top to bottom. Also, this effect only occurs if you don't pan with the car. You can totally eliminate the forward leaning effect by panning.

As I mentioned, a focal plane shutter mechanism moves the curtains at a very precise speed. This speed is determined during manufacturing of the mechanism and is governed mostly by how recently the shutter was designed. In older 35mm cameras, this speed was 1/60th second, but with time, shutters got faster and faster, and in the new D300 this speed is 1/320th sec. And those race car wheels lean much further forward with an older 1/60th shutter than with the D300 1/320th shutter.

Once you understand how the shutter works, you can begin to see what needs to be done to synchronize the flash. Depending on the design of the flash, the length of a full power flash will vary. In an SB800, the maximum flash lasts about 1/1050th sec.

The key point is that the flash, firing at 1/1050 sec is much faster than the curtains which move at 1/250th sec on the D200 (or 1/320th sec on the D300), so to synchronize the flash it must fire at some point after the first curtain has fully opened, but before the second curtain begins to close, so the sensor is completely exposed to the light from the flash coming through the lens. You may also see that you have a choice of when you fire the flash; either right after the shutter opens (front curtain sync) or right before the shutter closes (rear curtain sync). The timing of the flash has profound effects on the light trails when using very long shutter speeds. It will place the light trails either in front of the subject when using front curtain sync and behind the subject when using rear curtain sync. Rear curtain sync usually looks more natural for light trails.

Up to this point I have been only discussing the normal flash sync. However, Nikon has added a mode called 'Auto FP High Speed Sync'. You select this mode in the menu on the camera. In this mode, the flash duration is stretched so that it fires continuously for the full curtain travel time, ie, 1/250th sec on the D200. So, the the flash pulse that normally takes 1/1050th sec is now stretched across 1/250th sec. The official name for this operation uses the word 'Auto' in front of 'FP High Speed Sync', because in this mode the flash will sync normally and fire normally below the flash sync speed, but it switches automatically into High Speed Sync (stretched flash) when the shutter speed is set higher than Normal Flash Sync Speed.

This all sounds like a pancea until you find out that stretching the flash pulse and using higher shutter speeds causes the power of the flash to be reduced from what it normally is. In fact, the times where you would really like to be able to use a mode like this, like fast action sports in bright daylight, you usually can't, because the high speed flash sync mode is not powerful enough.

In fact, the power becomes less and less as you increase shutter speed, because the slit gets narrower and narrower. So, in this mode the flash is dependent on aperture and shutter speed, and if the shutter speed is increased to 1/8000th sec, the power gets so low that a subject would have to be less than four feet away.

However, when stopping action is not the goal, and a really high shutter speed is not required, then FP Sync can be very useful. In fact, Auto FP is excellent for shooting portraits in bright daylight. Then, you can use a shutter speed well above the normal sync of 1/250th coupled with a wide aperture to blur the background which greatly helps isolate the subject. I typically use camera A mode, ISO 100, FP Sync, f/ 2.8 and 1/1000th in bright shade, and I get a flash range of about 10-15 feet. If I am in really bright light, like on the beach, the shutter has to be around 1/1600th, and this reduces the range to about 8-10 feet which is still reasonable for portraits.

If a greater distance than 10 feet is needed in bright daylight, there is no choice except to use regular flash sync and accept the requisite smaller aperture. This happens frequently when shooting a party outdoors, where I typically shoot in camera S mode, regular flash sync, ISO 100, 1/250th, and f/7, and I get acceptable fill out to 20 to 30 feet or so.

9. TTL and TTL-BL Study

2008-03-06T13:21:00.000-08:00

After several discussions on Nikonians Forums concerning how TTL and TTL-BL work with respect to off-center subjects and distance, I thought I'd do a study to really find out for sure. Until now, all my comments on this topic have been based on what I've seen in my wedding photography. Now, I can say for sure exactly what is taking place.

This is more of a practical study than a scientific one. I have mostly been looking for effects, and not setting up control shots for precise measurements. I'm interested in how to use this knowledge to take better pictures, not for a review of the system for a specification.

The first thing I wanted to experiment with was the issue of how TTL and TTL-BL differ when shooting a light subject on a dark background at the same distance.

All the images in this study were shot with the following:
D200 Camera on a tripod
SB800 flash in the hot shoe with the head aimed forward
Nikkor 17-55mm f/2.8 AF-S DX lens
Matrix metering
Auto WB
f/2.8 and 1/60th sec unless noted otherwise
The location was indoors during daytime with blinds closed and a low intensity incandescent ceiling lamp on
AF-S Focusing unless noted otherwise

How Subject Position Affects Flash Power

In this set of tests I used a dark blue bathroom towel as a background and I placed a round white paper targets (the 'subject') at various places within the frame to see how the power of the flash would adjust. The reason I used a bathroom towel was that the texture of the towel provided a good AF target and I consistently got sharp and solid focus.

Image 1 Reference image, no flash , f/2.8, 1/1.6 sec

In Image 1, I used available light only, just as a reference. Notice the color of the bright 'blue' towel has shifted due to the Auto WB, which tried to make it 18% gray. The histogram shows a narrow peak representing the narrow range of brightness values for this image.

Image 2. Flash TTL, f/2.8, 1/60th sec

Image 2 is also for reference. It also hows how the Auto WB gets the color right when flash is used. The histogram shows a similar pixel distribution as in Image 1.

Image 3. Flash Off, f/2.8, 1/60th second

Image 3 is another reference image using the same f/ stop and shutter that will be used for the rest of the images in this study. Notice that the ambient is so low that will not be a factor in any of the remaining images in this study.

Image 4. White disk top right corner, Flash TTL

Image 4 shows the effect on the histogram of introducing a white disk of about 3 inches in diameter into the upper right corner of the frame. If you compare this to image 2 you can see that the background (the towel) has gotten slightly darker as a result of the flash metering system 'seeing' the white disk. Now, carefully look at the histogram, and you can see that the main hump of pixels representing the dark blue towel has moved left, and there is a small hump of pixels bunched against the very right edge. This small hump of pixels is the white disk, and its presence as a highly reflective subject has forced the flash power to decrease, which is why the background (the towel) got darker. The fact that the pixels for the white disk are climbing the right edge means that the flash has blown them out. Those pixels are overexposed and if there were any detail in such an area, it would be unrecoverable.

Image 5. Disk in upper right corner, Flash TTL-BL

Image 5 shows the effect of using the flash in TTL-BL mode. Comparing to Image 4, which used TTL, we see that TTL-BL sees the disk and tries to match it to the background. The result is actually quite good due to the fact that TTL-BL also uses distance information from the lens to set the power. This all has the effect of making the entire image much darker and the background is nearly black. The pixel hump representing the background has moved nearly to the left edge of the histogram, while the smaller pixel hump representing the disk has also moved left so it is no longer climbing the right edge. There are no blown out pixels in this image and it would be usable.

I am skipping Image 6 and 7 where I moved the disk to the bottom left corner, because the brightness and histogram distribution were essentially identical to Images 4 and 5.

Image 8. Disk centered, Flash TTL

In Image 8, I moved the disk to the center of the frame, and used the flash in TTL mode. You can see that now the flash metering system is more greatly affected by the disk more than it was in Image 4 when the disk was in the corner, and it decreased the flash power further. You can tell by the left shift of the background hump (towel pixels), and if you look closely, you will see that there is one thin line of pixels climbing the right edge indicating that the disk is totally blown out. So, even with the substantial darkening by moving the disk to the center, it is still not enough to prevent the disk from being totally blown out. This is because the disk is smaller than the center weighted measurement area, which allows a considerable portion of the dark towel to influence the metering. For proper monitor preflash measurements, the subject must be centered and occupy 18% of the frame. In this case it occupies only 7% of the frame, so lots of the dark towel is getting involved.

Image 9. Disk in center, TTL-BL

In Image 9, I switched the flash to TTL-BL and you can see that once again, the flash metering system tried to reduce the brightness of the subject (the disk) to match the brightness background (the towel). This moved the histogram to the left so that it looks essentially identical to Image 6, and indeed it should, because both were balanced to the dark towel. The fact that the disk was in the center made no difference. To state this another way, TTL-BL works differently when the background is darker than the subject. This is an area that is the subject of much research, and newer cameras than my D200 will likely deal with the dark background problem much better. It is my understanding that they are working on a way to switch to regular TTL when the background is dark. If this works out, it may obsolete regular TTL.

Focal Distance Tests Using a DX lens

Image 10. Disk in center, Flash TTL-BL, focus distance set manually to infinity

In Image 10, I am studying the effect that focal distance has on flash power.

I used TTL-BL and manually refocused the lens to infinity, and you can see the dramatic effect. Obviously, the image is no longer sharp, because it is way out of focus. The lens is a DX lens, which reports its focal distance, and it told the flash metering system that the subject was at infinity, so the flash increased to a very high power for the shot. I have no way to tell exactly what power the flash chose, but you can see that the histogram was shifted so far to the right that the background pixel hump is now in the center, and if you look really close, you can see the subject pixels are climbing the right edge compressed into a thin line, totally blown out.

Image 11. Disk in the center, TTL, focused at infinity

In Image 11 I switched the flash to TTL and refocused the lens to infinity. If you compare Image 11 to Image 8 you will see that they are essentially identical in brightness (not image sharpness), which indicates that the flash power was not affected by changing the the focal length of the lens. To state this another way, distance is included in the flash power calculation when using TTL-BL but not when using TTL.

Conclusions

1. In TTL flash mode, the subject reflects more light back to the flash metering system when the subject is centered. This is consistent with the comments I made in my earlier blogs that the flash monitors the center weighted area of the frame. I have shown through this study that the flash power is influenced more strongly by the subject when it is centered, but it is also influenced to a lesser degree all the way to the edge of the frame.

2. It is totally clear that in TTL mode the subject will often be overexposed if the flash ev is not reduced. I have found in my own wedding and events work that the flash works best when the flash compensation is set to about -1.0 ev for indoors.

3. TTL mode is NOT influenced by the distance that the lens reports.

4. TTL-BL mode is heavily influenced by the distance that the lens reports. I did not do a full study here, so I don't know exactly how much it increased, but by rough comparison with the flash in manual mode it appears to have increased to at least 1/2 power if not higher. This further implies that whenever we use TTL-BL mode we must make sure the distance from the camera to the subject is equal to the distance of the flash to the subject. In other words, don't use TTL-BL with the flash on a flash cable unless it is kept at the same distance to the subject as the camera.

5. It is totally clear that when the background is darker than the subject TTL-BL reduces the power of the flash in an attempt to balance the subject to the background. This works OK much of the time, but it is still best to use TTL indoors, where the subject is usually brighter than the background. However, I expect this conclusion to change as the newer versions of TTL-BL emerge on newer cameras.

8. How the Built-In Flash Metering Works

2008-02-29T19:23:00.000-08:00

Here is how the metering works when using the Nikon Built-In Flash.

When you select camera Matrix metering the built-in flash automatically switches to TTL-BL mode.

The question I will try to answer is: Why does it do this?

Well, when you use Matrix metering, the camera first meters the brightness of the ambient light of the entire frame fairly evenly from edge to edge. Then, when a subject is placed in a small portion of the center of the frame, you can see that it does not cause a significant change to the matrix measurement made by the camera. In other words, in matrix mode, the measurement that the camera makes is a very good approximation of the background ambient light BEHIND the subject.

This approximation concept is the fundamental basis for the TTL-BL mode!

Next, the built-in flash fires its preflashes, and it measures the strength of the reflection only in the center portion of the frame, because it is trying to measure the reflection from only the subject. In other words, the flash metering system always assumes there is a subject centered in the frame when it fires its preflashes.

Then, the flash computer compares the brightness of the subject (based on the reflected preflash from the center of the frame) to the brightness of the background (based on the matrix measurement of the entire frame from the camera), and it computes a flash power that will make the subject brightness equal to the overall scene brightness. Then the shutter is opened and the main flash is fired and the picture is taken.

Now, for the pop-up flash Nikon wanted to simplify things, so they forced the flash to always operate in TTL-BL whenever the camera is in Matrix mode. This relieves a novice photographer from ever having to make the fairly complex decision of when to use TTL mode and matrix together, which is allowed when using an external flash in the hotshoe.

Also, it's important to understand that the choice of the camera metering mode has absolutely no effect on the way the flash meters the preflash reflection. It always looks at the center of the frame regardless of what camera metering mode is chosen.

Now, if you switch the camera to spot metering, the camera now only measures the brightness of the subject due to ambient light; ie there is no background metering data, so obviously there is no way for the flash to work in TTL-BL mode when the background information is missing.

So the flash automatically switches to TTL mode, and no additional metering information is sent from the camera to the flash.

In TTL mode the flash uses only its own reflected preflash information, to set a flash power to make the brightness of the center of the frame (the subject) be a 'standard' brightness. This 'standard' brightness is adjusted in the factory to properly expose a subject of normal reflectivity placed in the exact center of the frame.

The size of the flash metered area is exactly the same whether in TTL or in TTL-BL modes. The size of the camera metered area changes depending on which camera metering mode is chosen.

Now, the pop-up flash can also work in TTL-BL when the camera is in Center Weighted mode, but this will only work correctly under certain circumstances. One is when the subject occupies only a very very small portion of the center of the frame, so that the camera metering information that is sent to the flash contains at least a little background information. Then the flash computer runs the TTL-BL calculations assuming the data from the camera is background information. It doesn't work very well most of the time, because the subject usually occupies too much space in the center of the frame and affects the background information substantially.

In summary, you may be able to see that there are a few situations that won't meter very well when using the built-in flash coupled with matrix metering, because you are forced to use TTL-BL. In fact, indoors the built-in flash will often work much better if you select Spot metering which forces the flash to function in TTL mode. Matrix mode will usually work better outdoors than indoors, because the background outdoors is generally brighter than the subject, and with the flash in TTL-BL the flash will add brightness to the subject to make it equal the background. Matrix and TTL-BL will not work well if the subject is brighter than or equal to the the background. That's when you need to switch to Spot and TTL.

This last situation highlights one of the big advantages of an an external flash, where you can leave the camera in Matrix and switch the flash manually to TTL.

7. How to Shoot Large Groups with Nikon CLS

2008-02-21T17:01:00.000-08:00

Shooting a large group, say 40 people or more, using only two Nikon SB800s in umbrellas is a challenge, but you can get decent results if you do it right. SB800s are fairly low power flashes for this type of work, so placement of the equipment is crucial. Here is an example of a recent group shot I took with just this equipment:

Notice that this image is far from perfect, but it is acceptable. It clearly shows some of the standard problems you usually run into with a large group and small flashes.

I used my camera in Manual mode and the flashes in TTL mode. I used the pop-up flash as the Commander with a Nikon SG-3IR filter in front of it. The SB800 only responds to IR light that enters through the small round red window, and since the the pop-up Commander puts out both visible and IR, the SG-3IR is useful to supress the visible portion of the light to help keep people from blinking.

In a large room like this, where there are no walls or ceiling from which the preflash pulses can reflect, the SB800 flashes in umbrellas must have their round red IR windows facing the Commander, or the preflash pulses from the Commander will not control or fire them.

As you may be able to tell, the ambient light was strong, and it was some weird mercury vapor color so there was no way to match the flash to it. Therefore, there is a slight color shift between the front row and the back row indicating that the front row was lit mainly by the flash, but the farther away from the flash you get the more the ambient had an effect.

Holding focus from the front row to the back row is always a challenge. You address this by creating as much depth of field as possible by using as small an f/ stop as possible. However, the smaller the f/ stop you use, the more power is needed from the flashes to provide enough light.

You also want to have the rows stand as close together as possible. Notice how closely the rows are standing to each other in this image.

I used f/5.6 on this shot, because that was the smallest aperture I could use and still get adequate lighting from my flashes. I set the camera in Continuous Servo AF with the switch on the front of the camera, CSM a1 to FPS Rate (ie, Release Priority), and CSM a6 AF-ON Only (ie to focus only when the AF-ON button is pressed). This allows me to aim at the second row, push the AF-ON button to focus (or use manual focus if I want to), and then recompose to place the group in the frame as I want it, and then push the shutter button to take the shot, and the green focus light doesn't have to be on for the shutter to release. This shifts the focus toward the middle of the group so depth of field is optimum for getting everyone sharp. F/5.6 is barely adequate for holding focus across a group this large. F/8 would have been more desireable if I were using say three or four SB800's.

I put my two SB800s in reflecting umbrellas on my ten foot stands extended as high as they would go, and angled them down so the light would come from above the group as much as possible which minimizes the fall-off of light from front to back.

Also, for a large group, you must place the umbrellas as close as possible to the group to minimize the Square Law light fall-off. In this case, I placed the umbrellas about three feet on either side of the camera and moved as close to the group as possible. This meant I needed to use the 17mm wide angle position on my lens.

I also had to leave some room at both sides of the frame for cropping. If you fill the frame from edge to edge, you will be able to print an 8x12 print, but not the standard 8x10. I have never been able to find a low cost 8x12 frame anywhere, but 8x1o frames can be bought at Walmart for $6.00. When I shoot a group, I simply look at the frame as though it is an 8x12 print and remember to leave one inch on each end.

I used a fairly slow 1/60th second shutter to allow the background to show.

Then there is the reflectivity issue. In this shot the group was mostly dressed in all-black, so the problem I had was to get the remote flashes to fire at the correct power. Black does not reflect light very well, so the flash metering system thinks it needs to increase the flash power high enough to create the reflection. This will totally blow out the faces. In this case, I decreased each remote channel on the Commander by -1.3 ev, and that corrected this problem.

Once I got the flash power where I wanted it, I locked it in with the FV Lock button. This button fires the preflash sequence and locks in the resulting power as determined by the flash metering system. This step is very important if you will be taking several group shots and you want them all to look the same.

One more problem with shooting groups is how to get everyone's eyes open. I tell everyone to blink quickly for a few seconds, while I am counting to three, and then I say 'Hold your eyes open' just before pushing the shutter. That works pretty well, but I also take at least three shots of each pose, so if there is a person with eyes closed, I can copy just the eyes in photoshop from a good image and paste them onto the final group image.

Additional Information On Shooting the Group Shot Above:

When you use SB800s in remote mode the red AF-Assist lamp in the SB800 doesn't come on, so then the choice of AF-S versus AF-C is only based on focus performance in ambient light.In this case, I was using the pop-up flash as a commander, and for group shots I usually keep the white AF Assist lamp in the camera turned Off with CSM a9, because it causes some people in the group to squint.

Obviously there has to be enough ambient light for the focus without any AF assist lamp. If there isn't enough light, then I normally go back to an SB800 mounted on the camera (or on a bracket) in commander mode, and switch to AF-S so the red AF Assist Lamp will come on.

However, assuming enough ambient for focus, in addition to AF-C, I have set the focus to use only the AF-ON button, and the priority on FPS (Release Priority). This way pushing the shutter button no longer has any effect on focus.

The main thing I am trying to achieve is accurate focus on a specific point somewhere in the middle of the group. I have found that with AF-C mode it is simple to hold down the AF-ON button and move the camera aim around and observe various points within the group pop into focus. Then, when I get the point I want, I release the AF-ON button and the camera stays focused at that spot while I recompose to place the group within the frame where I want it. Then the focus stays fixed on that point for a series of shots. I also watch the green focus lamp in the viewfinder to make sure the camera thinks it is in focus when I think it is.

I could use AF-S mode in Release Priority too, but then to get the focus on exactly the point I want requires pushing and releasing the AF-ON button repeatedly, because in AF-S mode, once it achieves what it thinks is 'focus' it stops trying, and it usually doesn't end up focused on exactly what I want it on. I like the focus to keep 'trying' until it gets to what I want.

Another way to handle focus is to use the AF to do the initial focus and switch to Manual Focus once focus is achieved. Then focus remains fixed for the rest of the shoot.This is actually the way I do it most often when using a tripod.

And for this group shot, I used ISO 400 and WB on Flash. Sometimes I use Daylight WB when shooting flash, which will make the image slightly cooler. But I never use a custom WB when using flash, unless I use gels to match the flash to the ambient. Then, I use PRE to set a custom WB that matches the ambient.

6. Sequence of Events for TTL Flash

2008-02-18T15:08:00.000-08:00

Let's assume we have these initial conditions:
normal indoor ambient lighting, camera A mode at f/3.5 and ISO 400.

Flash in Hotshoe

When you shoot a flash picture with the flash in the hotshoe, the following flash sequences take place:

1. Flash fires its preflash sequence.

2. The flash metering system measures the reflected light in the center of the frame from the preflashes and determines the power for the main flash.

3. The calculated power for the main flash is then modified by flash compensation that is set on the on the camera or the flash. This includes the dedicated flash compensation button on the camera, the camera ev setting, and the compensation setting on the flash itself. The power for the main flash is now finalized.

4. The shutter opens and the flash fires its main flash.

Remote Wireless Flash

Now, when the flash will be fired wirelessly a lot more commuication takes place during the preflash sequence. Here are the steps:

1. The Commander fires a control flash sequence that tells each of the remote flashes to fire its preflash sequence.

2. Each remote group fires its preflash sequence one at a time. If there are multiple flashes in a group, they will all fire their preflash sequence simultaneously.

3. Based on some very valuable work by Hal Becker (my fellow Moderator 'HBB' at Nikonians Speelight Forum http://www.Nikonians.org/forums), we now know that the flash metering system reads the reflected light from each group individually and immediately calculates and sends power information back to each group. Note that this system is not as smart as we originally thought. Each group's power is set irrespective of other groups that may be in use, and there is no adjustment for a situation where multiple flashes hit the subject in the same spot. I believe that this is why each group alone is purposefully set to underexpose the subject. Then, when multiple flashes are used, like in a studio, the subject is usually exposed properly.

A power setting is also set into the commander unit if it contains a flash as well. Multiple flashes in the same group will all receive the same power settings.

4. The shutter opens.

5. The Commander fires another control sequence telling all groups to fire at the power decided in step 3.

Also, the preflash sequence gets visibly longer when more remote groups are being controlled, and it becomes more important to block the visible light from the commander to avoid the preflash causing the subjects to blink.

If you want to see the preflash separately from the main flash, use the FV Lock button. You will see that when the flash is in the hot shoe, the preflash sequence is very short. As soon as you use the flash wirelessly, however, the preflash sequence gets visibly long due to all the extra communication that must take place in this mode.

Also the preflash sequence delay can cause problems when shooting action, so it is best to use FV Lock first which removes the preflash delay.

Next: How to Shoot Large Groups using Nikon CLS

5. A Study in Camera Compensation when using Flash in TTL Mode

2008-02-10T10:25:00.000-08:00

The SB800 Flash Instruction Manual says that when using the flash in TTL mode, adjusting Camera Compensation will lighten or darken the entire image, which means it must increase or decrease flash power, so I thought I examine that subject.

Conditions:

Normal indoor ambient lighting
Subject five feet away is a small statue of a boy on a horse placed in the center of the frame to be sure the flash metering will measure it properly
Background is a bookcase 10 feet away
Camera in Matrix metering
Flash in TTL mode

Note: The images in this study are directly from the D200 camera without any modification except compression to make them suitable for uploading.

Image 1: Flash OFF, Camera A mode, 1/25th, f/2.8, Camera ev=0
I shot Image 1 with the flash Off using camera A mode with f/2.8 selected. I could see in the viewfinder that the camera automatically selected 1/25 shutter. Notice that the image is properly exposed with pixels in the histogram extending all the way from black shadows on the left to bright highlights on the right and there are some shadows on the face and is approximately the same brightness as the background. The yellow color is due to the incandescent lighting and the poor Auto White Balance function on the D200. I will not try to correct or balance the color during this study.

Image 2: Flash On in TTL mode, FEC=0, Camera A mode, 1/60, f/2.8, Camera ev=0 Note: FEC means Flash Exposure CompensationIn Image 2 the flash was used, and as soon as the flash was turned ON, the shutter changed to 1/60th, which is the lowest available flash shutter setting as determined by CSM e2. Note that the faster shutter darkened the background a bit compared to Image 1, which is to be expected. You can see how the dark pixels are bunched up against the left edge of the histogram, meaning that detail is being lost in the shadows. The hump of pixels on the right come from the subject. Note that the histogram shows that there are no blown out pixels (ie, climbing up the right edge), but the subject still looks too bright for the scene. This happened because the flash metering system measures a little too 'hot' on my D200/SB800 combination. I think this is a very common thing on the D200/SB800 combinations. That's why I normally dial in -.7 ev on the flash for indoor shots (but this is a sidebar and not the main subject of this study).

Notice also that the color of the bookcase in the background is still yellow since it is lighted mainly by the ambient incandescent lights while the subject is much whiter since it is lighted mainly by the flash.

Image 3: Flash ON in TTL mode, FEC=0, Camera A mode, 1/100th, f/2.8, Camera ev = -2 In Image 3 I decreased the camera EV to -2.0. You can see that both the background and the subject got darker (just like the SB800 book says they should). You can also see that the hump of pixels on the right moved left just like the darker pixels on the left, piling more of them up on the left edge. The question I am studying is exactly how did the camera & flash achieve this left shift of the pixels?

Well, I observed that the shutter automatically increased to 1/100, which darkened the background. You would have expected that when starting at 1/60th it would have increased to 1/240th to achieve two stops darker. However, I have discovered that the camera knew the ambient was actually at 1/25, and 1/100 is exactly two stops darker than 1/25. Pretty smart system.

Remember that changing shutter speed does not affect the flash portion of the exposure, because the flash occurs is so short (less than 1/1000th second).

And you can see that changing the camera ev definitely darkens the subject as well, which means it has to decrease the flash power. That indicates that the camera compensation information is sent back to the flash metering system when shooting in TTL mode.

So, when you change the camera ev, it changes the camera f/ stop or shutter speed settings to adjust the background, and it sends the ev change to the flash to change the flash ev by that same amount to adjust the subject.

Image 4: Flash OFF, Camera M mode, 1/25th, f/2.8, Camera ev = 0

To further verify exactly what the camera ev settings tell the flash, I first put the camera in Manual mode and set 1/25 and f/2.8 and shot Image 4 above without flash as a reference. Note that Image 4 and its histogram looks essentially identical to Image 1, as it should, since the same camera settings were used.

Image 5. Flash ON in TTL mode, FEC=0, Camera M mode, 1/60th, f/2.8, Camera ev=0
Then in Image 5 I increased the shutter to 1/60th and fired the flash, and this image looks just Image 2, as it should.

Image 6: Flash ON in TTL mode, FEC=0, Camera M mode, 1/60th, f/2.8, Camera ev = -2
Now, in Image 6, with the camera in Manual mode, 1/60th, f/2.8, you can see the effect of setting camera ev to -2. Comparing with Image 5, the subject in Image 6 definitely got darker, and the histogram hump on the right moved slightly left, but the background didn't change. This proves that ev changes on the camera are sent to the flash to the change its power, and the reason the background didn't change is because ev changes to the camera will not affect the f/ stop and aperture when in camera Manual mode. In the previous example, the camera was in A mode, and camera ev changes will change the shutter speed.

Image 7: Flash OFF, Camera M mode, 1/250th, f/2.8, Camera ev= 0

I took this shot without flash as a reference image to show just how little ambient there is at 1/250th. 1/250th virtually takes the ambient light out of the exposure in normal indoor lighting.

Image 8: Flash ON in TTL mode, FEC=0, Camera M mode, 1/250th, Camera ev=0

In Image 8 I turned on the flash and took the shot. Notice the dark background, because the flash exposes the subject correctly, but only a small amount reaches the background five feet behind the subject (due to the square law - power decreases by the square of the distance).

Image 9: Flash ON in TTL mode, FEC=0, Camera M mode, 1/250th, f/2.8, Camera ev = -2
Then, in Image 9 camera ev was reduced by 2 ev and the brightness of the subject (power of the flash) definitely decreased compared to Image 8. The brightness of the background barely changed, though because not much of the flash power is getting back there. This is showing that you can adjust the brightness of the subject without changing the background by changing flash power.

Image 10: Flash On in TTL mode, FEC = -2, Camera M mode, 1/250th, f/2.8, Camera ev = 0

Lastly, in Image 10 I set the camera at 1/250, f/2.8, and ev 0, and I used the Flash ev on the back of the flash to -2 ev. As you can see, the image looks exactly like Image 9, and the hump of pixels on the right (representing the subject) is in the same spot on the histogram.

Conclusion:

This study proves conclusively that camera ev is sent to the flash in TTL mode and can be used to adjust flash power, just like the FEC button on the front of the camera, and the FEC button on the flash itself.

Next: Sequence of Events for TTL Flash

1. Nikon Flash - Two Separate Metering Systems

2008-01-31T09:22:00.000-08:00

After several years of using the SB800 with my Nikon D200 while shooting weddings and events, and after writing numerous posts about this on the Nikonians Speedlight Forum , I decided to consolodate my thoughts in this blog.

Note: I use the term TTL to mean the same thing as iTTL throughout this blog.

Look at the two pictures above. Notice that the first one appears as though the subject is in a cave. The background is black. But the second one has noticeable detail in the background. This blog post is about being able to adjust between these two extremes at will.

Flash photography is a pretty complex subject, and you have to study it a while to understand it. It is also a very rewarding subject and leads to much better pictures in many situations.

The first concept to wrap your head around is that when you take a flash picture it is actually a combination of two exposures; one from available light (also called ambient) and the other from flash. Already you may be able to see intuitively that flash photography will be easier in the dark, because the contribution from ambient is zero! This concept is critical to understand.

And, it is also probably intuitive that if the ambient light is bright, things can get a lot more difficult as you try to balance the two contributions to the image.

So, Nikon made things much easier for us by developing their camera and flash metering systems - these are two completely separate systems that are used together or separately, depending on how we set up our camera/flash.

So let's look at the simplest situation first; a dim room where there is only a little ambient light; like what you would find in a typical indoor home setting at night. If you take a picture in such a setting without flash, in one of the auto modes, say A mode, a typical aperture/shutter would be about f/4 and 1/4th sec at ISO 100.

Now, put the camera in manual mode and increase the shutter to 1/80th sec and leave the aperture at f/4 and shoot another shot. The picture will be very very dark - to the point that the image is barely discernable.

Now, turn on the flash in TTL mode and shoot that same shot, leaving the shutter at 1/80th and the aperture at f/4 (still in camera manual mode). You will see that whatever is in the center of the frame will be properly exposed by the flash. This is because the flash metering system handled the exposure and added just the right amount of flash power to get a proper exposure. It still may not be exactly the right exposure for other reasons, but I'll discuss that in a future blog entry.

Now, place a subject (a person is perfect for this) relatively close to the camera (say 5 feet) with a background behind the subject (say about 15 feet away), still in typical home indoor lighting. Make sure the subject is in the center of the frame and shoot a flash shot in straight TTL (not TTL-BL) leaving the camera settings at f/4 and 1/80th sec. You will find that again the subject came out properly exposed, because the flash metering system handled the exposure. But the interesting thing is that the background behind the subject is back to being very dark, just like in the shot without flash. This is because the flash power decreases very quickly from the distance of the subject to the background and it barely brightens the background at all.

Now, decrease the shutter speed to 1/10th sec and shoot again. The background will be brighter, but the subject will be the same brightness as before. Also, the subject will be sharp, but the background may also show some motion blur, because 1/10th sec is too slow to hold the camera perfectly still (unless you use a tripod). Since the flash was primary on the subject; ie, the ambient was overpowered by the flash, and the flash duration is normally faster than 1/1000 sec, the subject will be sharp with no motion blur.

This shows you that in a dim room, the flash exposure of the subject is controlled by the flash metering system and the background exposure is controlled by the settings on the camera. You can adjust them separately! This is the key behind using the flash in TTL mode and the camera in Manual mode.

Even when the ambient lighting is a little brighter than this, you can still use this technique if you stop down the camera aperture enough to make the flash the primary light on the subject, which takes the ambient light out of the equation. But then your background becomes dark. And if you try to brighten it by decreasing the shutter speed, then the bright ambient light will begin to affect the subject brightness too, and you begin to risk overexposure of the subject. In this situation, shutter, aperture, and flash power all affect the brightness of the subject. It's usually best to simply live with a dark background in this situation and make sure the flash is primary on the subject.

Now, if you are in bright ambient light (like outdoor in daylight), it is simply too bright to be able to take the ambient out of the equation by stopping down the camera, so now you have to balance the flash and the ambient. If the flash contribution to the shot will be less than the ambient contribution, then it is called Fill Flash. When there is a lot of ambient, the background and subject are no longer able to be adjusted independently, and camera manual mode becomes difficult to use. This is when you want to switch the flash to TTL-BL and the camera to one of the automatic modes (S, A, or P). The camera will then measure the ambient light to set its aperture and shutter and send this information to the flash, after which the flash metering system will set the power of the flash to make the subject the same brightness as the background.

If the ambient is extremely bright, like direct sunlight, then it's usually best to select P or S mode (and TTL-BL), so the aperture will automatically stop down to avoid overexposure. In A mode, the shutter will increase to reduce the exposure, but it will be limited by the flash sync speed of 1/250th (D200) and that is often not high enough for bright light if a wide aperture is selected.

One caviat for TTL-BL: the subject must be darker than the background for TTL-BL to work properly. The flash can only brighten the subject to balance it with the background; it can't make the subject darker. If the subject is brighter than the background to begin with, then you probably are best off not using flash. However, there can sometimes be slight shadows on the subjects face that can be 'lifted' with a very slight amount of flash in TTL mode.

Go to next post

2. Nikon TTL Flash Metering System

2008-01-30T11:34:00.000-08:00

Conditions: Flash in iTTL Mode - not iTTL-BL Mode.

(TTL means iTTL in this blog)

It is very important to remember that the Nikon TTL Flash metering system is not actively coupled to the Camera metering system. Even though these two systems use the same metering sensor in the camera, they act independently. This is definitely not intuitive and not explained well in any documentation that I've read. It is alluded to in the Nikon CLS pamphlet when it talks about using the FV Lock button to meter a subject that will be off-center when the background is dark.

Note: The TTL-BL flash metering system is actively coupled to the camera metering system, and I will talk about that in my next post.

The picture above was taken in nearly total darkness in camera Manual mode with the shutter at 1/8th second and the aperture at f/2.8. Notice how the flash system handled the entire exposure of the subject, while the camera system handled the the exposure of the background. You know this because of the shift in white balance from the subject to the background. Notice that the background is very yellow and the subject color is much whiter. This is because the flash was the only contributor to the exposure of the subject, and the background was lit by incandescent lights. The flash was so weakened by the time it reached the background that it did not contribute to the background at all.

The Camera metering system does not measure the amount of regular TTL flash that will be added to the exposure of the subject. On the D200 and all cameras prior to the D300/D3, the camera does nothing to help you with this. If you are shooting regular TTL flash in bright ambient light, you risk severe overexposure if you do not reduce the camera exposure when you turn on the flash.

On a D200 camera you can easily demonstrate this problem by taking a picture, with and without flash, with the shutter set at a fixed amount. Set the camera on S mode and the shutter to 1/80th and let the camera pick the f/stop. The f/ stop that the camera selects is the same whether the flash is on or off. This means that whatever ambient light is reflected from the subject will be further illuminated by the the flash, and if the ambient is already enough to properly expose the subject, the flash will cause overexposure.

However, on the D300 and D3 and newer cameras, Nikon has made a significant improvement to help with this. While the camera metering system still does not meter the amount of TTL flash that will be added to the exposure of the subject (like it does in TTL-BL mode), the newer cameras automatically reduce the camera exposure in bright ambient light when the flash is turned ON. For instance, in the camera A mode, in bright ambient light, the shutter speed is increased when the flash is turned on. The amount of the increase is dependent on how bright the ambient light is. This process is similar to the process that the photographer was required to do with flash compensation prior to the D300/D3.

The Flash metering system just fires the preflashes and looks for the reflected light in the center weighted frame regardless of which camera metering system you are using or what the camera f/ stop and shutter is set to. The flash metering area is affected most by centered subjects and the metering sensitivity decreases as the subject is placed farther and farther towards the edge of the frame. At the edge of the frame, the subject has much less effect on the flash metering. Whatever light reflects back from the preflashes in this center-weighted fashion is what determines the power of the flash. You can see that if the subject will be way off-center, your flash power will likely end up way too high as the flash attempts to light the background in the center. This is why the FV Lock function was invented, which I have written about here: http://nikonclspracticalguide.blogspot.com/2008/01/4-so-what-is-flash-value-lock.html .

So, you can reason that the ambient light reflected from the subject is essentially added to the exposure once by the Camera metering system and once by the Flash metering system and overexposure is often the result if the ambient light is strong.

Here is an example of overexposure with flash when ambient is strong.

One important operational consideration that should be drawn from this is that when the ambient light is bright, it is usually best to turn down the flash using flash compensation to about -1.7 ev to start with. This will usually avoid overexposure on especially the subject's face.

Note: This is what the newer cameras (D300/D3 and newer) do for you automatically, except they do it by reducing the camera settings, which reduces the ambient portion of the exposure. The thing that is not so nice about this is that if you use regular TTL outdoors in bright daylight for fill, this automatic reduction in camera exposure will also make the backgrounds darker. So this improvement is really only a band aid that protects the unknowing photographer from severe overexposure. The best way to shoot fill using regular TTL is to use the camera in Manual mode where you have full control of the shutter and aperture. Then, you reduce the flash power with the flash compensation as I mentioned above.

This is why Nikon invented TTL-BL, which I'll talk about in my next post.

3. Nikon TTL-BL Flash

2008-01-21T13:07:00.000-08:00

The most advanced flash mode on the Nikon flash is TTL-BL. Originally, the BL meant BackLit, but Nikon marketing gurus changed it to mean BaLanced Fill. (Why didn't they change the initials to TTL-BF?). The word BackLit suggests the situation for which this mode is most useful.

The photo on the left is a good example of a subject being backlit by a brighter scene.

I have discussed in the previous blog entry that in the TTL mode the Camera Metering System is entirely separate from the Flash Metering System, and that they work independently. Well, in TTL-BL, the same two metering systems do communication with each other.

First, let's discuss what Fill Flash is and when it is needed.

Fill Flash is needed any time the background (also called ambient) is brighter than the subject and this often occurs outdoors on a bright day. In this situation, if you look closely at the subject's face, you will see shadows in the eye sockets, under the nose, and under the chin. When you shoot this type of shot, it will look much much better if the flash is used to 'fill' the shadows. Another situation would be if the background behind the subject was the bright sky. Again, flash should be used to 'fill' the subjects face and make it balance with the bright sky. One more situation would be when indoors during the daytime, and you want to shoot the subject in front of a window with a beautiful brightly lit scene behind (like the picture above). Again, the flash should be used to brighten the subject's face to balance with the bright scene behind.

These are the situations where Nikon's TTL-BL mode works best.

In the above example image the subjects are in front of a window and I wanted the outside to be exposed properly by bright ambient light, while I needed lots of fill to bring their faces up to be slightly less bright than the outdoors. I put my D200 in Manual mode, ISO 200, and used the built-in light meter to set f/3.5, 1/160th, and TTL-BL with -1.7 ev worked great in this situation to just bring up their faces to what seems a natural brightness.

Here are the steps that the camera and flash do for you as you push the shutter:

Given: Flash in TTL-BL, Camera in P mode, matrix metering, and AF-S focusing.

1. As the shutter is half-pushed, the focus system becomes active, the focus is achieved, and the overall scene is metered by the camera.

2. The data from the camera metering system and, the focal distance from the D or G lens are sent to the flash metering system. This is the only communication that takes place between the camera metering system and the flash metering system.

3. As the shutter is pushed down the rest of the way, the flash fires the preflashes, and the flash metering system measures the reflected light. This reflected energy is a secondary factor to the distance reported from the D lens in the TTL-BL equations. If a D lens is not being used, the preflash reflected power assumes a more important role in the TTL-BL equations. When using a D lens, the main purpose of the preflash is to set the white balance.

4. The flash metering system (which actually resides in the camera) then uses the distance information from the D lens and the data from the camera metering system, to determine the amount of added power required to make the subject brightness equal to the overall scene brightness. In other words, it adds light from the flash to balance the brightness of the subject with the ambient.

5. The shutter opens, the flash fires at the power level determined above, and the shutter closes.
Now, for this all to work well, a couple of key things must be considered. First, the subject (obviously) can only be brightened by the flash. There have been several advancements to TTL-BL since I originally wrote this, and now TTL-BL handles the situation fairly well when the subject starts out brighter than the background in low ambient. However, it is still best not to use TTL-BL if the subject is already brighter than the overall scene. Regular TTL usually works better.

Second, flash pictures usually look really odd when the subject is just as bright as the background. The subject seems to jump off the page unnaturally and it is obvious you used flash. Fill flash is supposed to be subtle, and when looking at the print it is usually best that you cannot even tell that flash is used at all. Consequently, it is often best to turn down the flash compensation by -.3 ev to -0.7 ev, so the fill is just enough to lift the darkest shadows on the face without looking obvious.

Notice that the flash never tells the camera anything about the power setting it has chosen. The camera sets its f/ stop and shutter (in auto modes) as if the flash wasn't even attached (with the exception of limits on shutter speed). The only coupling between the camera metering system and the flash metering systems is when the camera sends its metering information and focal distance (if using a D or G lens) to the flash metering system.

If you are using camera Manual mode, the flash system is not even told what f/stop or shutter you have selected. It assumes that you have zeroed the light meter in the camera and sets the flash power accordingly. It only receives the metering data from the camera metering system and uses that as the 'background' brightness. This is why it is often best to use camera P or S modes when shooting fill flash. They automatically zero the light meter in the camera using the aperture which avoids the overexposure that often occurs when using A mode.

Of course the flash also knows the f/ stop and ISO from the camera settings which are sent through the hot shoe.

Next: Flash Value Lock

4. So, What is Flash Value Lock?

2008-01-20T14:33:00.000-08:00

I've written in my previous posts about the fact that the flash metering system measures only the center of the frame. This means that if the subject is not in the center of the frame, the brightness of the subject will likely be wrong.

Above is an example of a subject that is not in the center. I used FV Lock to make sure the subject was exposed properly by the flash. I was outdoors at a reception on a pitch dark night, so there was zero ambient light, and I wanted the very dimly lit waterfall to come out in the background, so I used my D200 handheld in manual mode, ISO 400, 1/8 sec, f/2.8 and my flash in TTL.

I first pointed my flash at the subjects and pushed the FV Lock button to fire the preflashes and lock in the correct flash power. Then, I recomposed the shot to place the subject on the left 1/3 line (rule of thirds), and released the shutter. Notice that the subject is sharp despite the very slow 1/8th second shutter speed, because the flash stopped any motion. But, look at the waterfall. There is definite motion blur there, but I used f/2.8 to make sure it was out of focus so the motion blur from handholding the camera wouldn't show.

If I had just framed it as above to start with and taken the shot without using FV Lock, the flash would have metered the center of the frame, which was the waterfall about 70 feet away, and it would have set a very high power. The subjects would have been totally blown out.

One very important thing to know when using FV Lock is that once it is pushed the flash value remains locked at that power setting until one of three things takes place: 1) FV Lock is pushed a second time, 2) the camera is turned off, or 3) the light meter in the camera times out. Since the default light meter reset time is very short (only 6 seconds on my D200), you need to use your camera menus to extend the light meter timeout to as long as possible when you are using FV Lock.

Don't forget to return the light meter timeout to its default time after you are finished with FV Lock, or it will run your camera batteries down faster than normal.

FV Lock is very useful for shooting a series of shots of a group of people, so that all the images will look exactly the same from one shot to the next.

It is also useful for those rare people who are 'fast blinkers'. A fast blinker can blink at the preflash and their eyes will be closed by the time the main flash happens. Lot's of animals are fast blinkers, so FV Lock is also good for them.

Next: A Study in Camera Compensation while using Flash