Camera College: Autofocus systems
How your camera focuses, and how to get sharper results
Today’s cameras put a comprehensive set of autofocus controls at your finger (and thumb) tips, but there are essentially two simple steps you need to take regardless of how sophisticated or complex your camera is. First, set the focus mode to match the subject you’re shooting, then decide where you want the camera to focus.
There are typically three autofocus or AF modes available when you’re taking pictures through the viewfinder of an SLR or CSC: a single-shot mode for stationary subjects; a continuous ‘servo’ autofocus mode for moving subjects; and an automatic mode that switches between the two as and when the camera detects some movement.
Single-shot autofocus is the mode to select when you’re photographing subjects that aren’t moving. It won’t let you take a picture until the camera gets something in focus. Once it’s done this, the camera locks the focus distance in for as long as you keep a light pressure on the shutter release button, and the picture will be sharply focused unless you or the subject move. As the name suggests, continuous autofocus constantly adjusts the focus as long as you keep your finger on the shutter release. As a result, it’s the best mode to use for photographing moving subjects. The focus position is never locked, and the camera will let you fully press the shutter to take a picture even if the subject isn’t in focus.
A camera’s autofocus or AF modes control how the camera focuses, but the AF points you see in the viewfinder determine where the camera focuses – and how precise it is too. The greater the number of AF
points there are and the wider they’re spread across the picture, the easier it is to focus on subjects that aren’t in the centre of the frame. A densely packed array of AF points can improve your chances of taking sharp shots of sports and wildlife, as a moving subject is less likely to be in an area of the picture that isn’t covered by one of the points. It also means that you’re less likely to have to adjust the camera position to focus before recomposing a shot.
You can choose to use just one of your camera’s autofocus points or all of them. With all of the AF points active, the camera will automatically choose the focus point or points that correspond with the area it determines should be in focus. Typically, this is the closest part of a scene or subject, or the area of highest contrast . Automatic focus point selection can be effective when you’re photographing a subject against a clean background, such as a bird flying across a blue sky. However, if that bird passes against a more detailed background, there’s a chance that the focus system will lock onto that instead.
Manually selecting an AF point gives you precise control over what’s in focus, but it does mean that you have to keep the active AF point positioned over the subject.
The number of AF points and their precision is determined by the lens attached to the camera – or rather, by its maximum effective aperture.
A densely packed array of AF points can improve your chances of taking sharp shots of sports and wildlife, as a moving subject is less likely to be in an area of the picture that isn’t covered by one of the points
In your camera manual, you might see an f/stop mentioned in relation to the AF points, such ‘f/5.6 cross-type’ or ‘f/2.8 dual cross-type’. This is an indication of the maximum aperture required to activate additional levels of precision of a focusing point. It’s the maximum aperture of a lens that counts, not the aperture you set when you’re taking a photo: an f/2.8 lens can enable f/2.8-compatible sensors even if it’s used at f/11.
Some cameras have a centre AF point that works with lenses that have a maximum aperture as small as f/8. This comes in particularly handy when you’re using a telephoto lens with a teleconverter attached, as the effective maximum aperture will be smaller than it is with the lens alone.
There are a different set of autofocus options available when you’re using an SLR’s Live View mode, or if you’re using a mirrorless compact system camera (CSC) where Live View shooting is the only option. Here, the mirror assembly is locked out of the way – or in the case of a CSC, is non-existent – so the camera’s imaging sensor is continuously exposed to the scene. This means that the camera has to use the image captured by the sensor to drive the autofocus. This does mean that focusing is potentially more accurate, as what you see is what you get – with an SLR’s optical viewfinder, the image you see is not the final image captured by the camera’s imaging sensor.
Live View autofocus can include such options as face tracking, or blink or smile detection. SLRs may also offer a ‘quick’ mode, which temporarily moves the mirror back into position so that the camera’s AF module can be used to focus, before moving the mirror out of the way again. It can be a bit of a clunky process, and not quite as quick as the name suggests.
It can be harder to keep the camera in position – and the subject in focus – when using the Live View display, but a mirrorless camera equipped with an EVF enables you to support the camera better, and improves your chances of taking a sharp shot.
MISSED FOCUS Use continuous autofocus when you photograph fast-moving subjects, but be prepared to give your camera time to ‘warm up’ and track the action.