In “single AF” mode, the camera will focus when the shutter release button is pressed halfway. Some cameras offer “continuous AF” mode whereby the camera focuses continuously until you press the shutter release button halfway. This shortens the lag time, but reduces battery life. Normally a focus confirmation light will stop blinking once the subject in focus. Autofocus is usually based on detecting contrast and therefore works best on contrasty subjects and less well in low light conditions, in which case the use of an AF assist lamp is very useful. Some cameras also feature manual focus.

Autofocus Servo is normally triggered by switching focus to “AI Servo” on Canon, or “Continuous” on Nikon, followed by half pressing the shutter release. The camera will continue to focus on the subject as long as the shutter release will be half pressed or fully depressed (actually taking shots). The Autofocus Servo usually also puts the camera into “release priority” mode so that the camera will take the shot when the shutter release is depressed, regardless of the current AF status.

The picture above is a hologram AF found on some Sony cameras. This works by projecting a crossed laser pattern onto the subject. This bright laser pattern helps the camera’s contrast detect AF system to lock on to the subject. The system works well as long as the subject is large enough to be covered by several laser lines.

The minimum resolution (bit rate) of an ADC is determined by the dynamic range (accuracy) of the sensor. For example if the dynamic range of the sensor is 1000:1 (or 60dB) the ADC must be at least 10 bits which is 2^10 = 1024 discrete levels. This will avoid loss of information. Theoretically a 10 bit ADC is the perfect fit for this dynamic range (1000:1), a 12 or 14 bit ADC will not generate additional tonal information other than noise. However, in practice it makes sense to overspecify the ADC to 12 bits to allow for some margin of error on the ADC.  It is also useful to have extra bits available to minimize posterization or banding when applying the tonal curve to the linear data.
Often, marketing material advertises the bit rate of the ADC to suggest the digital camera or scanner is able to output images with a high dynamic and tonal range. From the above it is easy to understand that this is only true if the sensor itself has sufficient dynamic range. The tonal range and dynamic range can never be larger than the dynamic range of the sensor.
Digital SLR cameras have sensors with a higher dynamic range and are usually equipped with 10 or 12 bit ADCs. Normally such cameras offer the option to save the 10 or 12 bits of data per pixel in RAW because JPEG only allows 8 bits of data per channel.

What goes through your mind in the moments as you raise your digital camera up to take a shot and before you press the shutter? If you’re like many digital photographers you’re not thinking about too much – you just want to capture the moment and then move on. However getting in the habit of asking some simple questions can help take your images to the next level. Here are some questions to get in the habit of asking while framing your shots.

What story am I telling? – This is an important question and one that should help you to make any number of decisions in terms of composition, framing, exposure etc. In essence what you’re asking is “why am I taking this shot? What is it’s purpose and what am I trying to convey?” Is it purely a way to keep a record of a moment, are you trying to capture the emotion of a moment, is it possibly a shot to give to someone, is it part of a larger series of shots or will it be the only shot to commemorate the moment etc.
What is the visual focal point of this shot? – What will viewers of this picture naturally have their eye drawn to in this scene? Once you’ve identified this focal point you can think about where to place it in the frame (consider the rule of thirds for example).
What competing focal points are there? – Once you’ve identified what you do want your viewers eyes to be drawn towards and have placed it in the frame – scan your eyes over the shot and see if there are any competing focal points and ask yourself whether they add to or take away from the image? Secondary focal points can add depth to shots but they can also be very distracting and so you might need to reposition yourself or adjust your focal length and/or depth of field to accommodate or remove them from your shots. Also keep in mind that if your shot has more than one focal point that it might be worth taking two shots, one of each focal point, in order to keep things simple.
What is in the background and foreground? – One of most common places for distractions in digital photography is the background of your shots. Run your eyes over the space behind your subject to see what else is in the image (do the same for the foreground). Consider whether you want the background in focus or nice and blurry.
Am I close enough? – Another common mistake in digital photography is taking shots where your subject is too small in the frame. Shots that fill the frame with your subject tend to be much more dynamic and show a lot more detail of your subject. To get this effect you have the option of moving yourself closer, moving your subject closer or using a longer focal length to give the effect of closeness.
What is the main source of light? – Always give consideration to how your subject is lit. Without light you’ll lose detail and clarity in your image and your camera will have to compensate by doing things like increasing ISO and lengthening shutter speeds (which could lead to noisy and blurred images). What is the main source of light, where is it coming from, is there enough light, do you need artificial light sources (flash etc), do you need to stabilize your camera on a tripod to stop camera shake due to low light etc.
Is my Framing Straight? – It’s amazing how many otherwise good photos are spoiled by framing that is slightly offline. Sloping horizons and slightly leaning people or buildings should always be in the back of your mind to check.
What other perspectives could I capture this subject from? – Put 10 digital camera owners in front of a scene and most of them will take exactly the same shot from the same position. Make your images stand out from the crowd by challenging yourself to not only take the standard shots that everyone else will get but to find creative and fresh angles and perspectives to shoot from.
How would holding the camera in the other format change this shot? – Many photographers get into the habit of always holding their camera the same way (horizontally/landscape or vertically/portrait). While it’s OK to have a preference one way or the other it’s also worth remembering that changing the format can drastically change the impact of the shot. Don’t forget you can also hold your camera at an angle for an effective result too.
How will the eye travel through this image? – This is related to asking about focal points but gets in touch with the fact that while you’re photographing a still image your viewers eyes don’t remain still as they look at an image. People tend to follow lines and are attracted to shapes and colors so considering all of these different visual elements and cues can help improve your shots considerably.

Of course you probably won’t remember all the questions and you’re unlikely to go through each of them with every shot you take – however next time you head out with your digital camera concentrate on asking yourself at least one or two of them as you take your shots. As you do you’ll find that they become more automatic and in time you’ll naturally take digital photography shots that take into account all of these elements.

Distracting Focal Points – this can happen to everyone – you line up a shot of a friend to take a portrait and just as you press the shutter someone else pops into the frame with the intention of ruin your shot. The result is that the real focal point of the shot becomes this person which pops into the frame. This is an extreme example of distracting focal points in the background but it’s something that happens quite a lot.
Protruding Elements from Subjects Heads – this is something so common that I have to mention it. When shooting a portrait one of the common mistakes is for some background element to look like it’s sticking up out of a person’s head – like a horn. It’s often trees but could be anything. These shots can be quite comical but can also really throw the composition of a shot off.
Competing Lines – if your subject has lines in it and your background also has strong lines they can compete in such a way that the image becomes busy or so that the lines clash with one another.

There are a couple of strategies for dealing with distracting backgrounds:
Check your Background Before Hitting the Shutter Release: Always scan the background of your shots before taking a shot. Look for colors that don’t fit with the rest of the image, bright patches that might distract the eye, lines that clash, people that don’t belong etc.
Move Your Subject: This is a fairly simple technique but is probably the first thing you should consider. Quite often asking a portrait subject to take a step to the left or right will fix things either by putting the distraction behind them or by putting it out of frame.
Change your Shooting Angle: If you have distracting elements in the background of a shot but can’t move your subject another strategy is to move yourself and shoot from a new angle. This might mean rotating around your subject but could also include getting down low to make the sky the background or even getting up high and shooting down onto your subject to make the background the ground.
Using Aperture to Blur Backgrounds: One of the most useful things to learn as a way to combat distractions in backgrounds (and foregrounds) is to use the power of your lens to throw the background out of focus using depth of field. What you’re trying to achieve with this technique is a nice blurred background where you can’t really make out what’s going on there. The easiest way to do this is to use a wide aperture (the smaller the number the wider the aperture). The wider your aperture the more blurry your background should become. The quickest way to see the impact of this strategy is to switch your camera into aperture priority mode and to take a number of shots at different apertures. Start with an aperture of f/20 and work your way down – one stop at a time. Once you get down to under f/4 you’ll start seeing the background in your shots getting blurrier and blurrier.
Using Focal Length to Blur Backgrounds: Another way to help get your backgrounds nice and blurry is to use a lens with a long focal length. Longer tele-photo do help a little to get narrower depth of field. In actual fact the impact is smaller than it seems and the main reason for the change is that with a longer focal length the subject actually takes up more space in the frame. Using longer focal lengths does seem to have some impact and is worth experimenting with.
Place Subjects In front of Open Spaces: Placing your subject a long way in front of other objects will also help to make those objects more blurry. For example if you have the choice between shooting your subject standing right in front of a brick wall or standing in front of an open field – the open field shot will have a much more blurred background simply because the brick wall is just centimeters from your subject whereas an open field stretches off into the distance where everything will be out of focus.
Fill your frame with your subject: One of the most effective ways of removing distractions from backgrounds is to remove the background altogether by totally filling the frame with your subject. Get up close and/or use your zoom lens to tightly frame the shot and you’ll not only remove distractions but could end up with a high impact shot as well.
Make your Own Background: Sometimes there just isn’t any suitable background and so you might want to consider making your own. This could range from buying a purpose built studio background or simply buying some cloth to do the job for you. The other thing to keep in mind is that in many instances you can move things around in the background of your shots (especially if you’re shooting indoors).
Post Processing: There are numerous ways of editing a shot after you’ve taken it to get rid of distracting elements. These can include blurring techniques, actual removing of elements and replacing them and techniques such as selective coloring (making your subject stand out by making your background black and white).