Modern autofocus systems are fantastic. They can get a subject sharp in the blink of an eye – faster even. And if you’ve ever tried focusing manually you’ve probably realised that they’re usually more accurate than your eye in most situations. However, things get a little tricky with moving subjects and you have to make sure that you’ve set the camera to the correct mode.
The first thing you need to know is that most cameras have two autofocus modes, and many have three. The most commonly used mode is Single-Autofocus, or S-AF. In this mode the camera focuses the lens when you press the shutter release half-way. Once it has achieved focus it won’t adjust the focus again even if you continue to hold your finger on the shutter button and the subject moves. If you want the camera to refocus the lens you have to lift your finger off the shutter release and depress it again.
Single-AF mode is a great option for stationary subjects and when you want to focus and recompose an image. However, it’s not a good choice for sport and action photography, that’s where Continuous-Autofocus or C-AF comes in handy.
In C-AF mode the camera continues to focus the lens for as long as you hold the shutter button down. If you also have the camera set to continuous drive mode it will continue to focus the lens between shots as they are fired off in rapid succession. This makes it a great choice for shooting sport because the camera can keep the subject sharp as it moves towards or away from the camera – provided the correct autofocus point is selected.
Many cameras offer a third automatic focusing option called Auto-AF or A-AF. When this mode is selected the camera attempts to decide whether the subject is moving or not and switches between Single-AF and Continuous-AF mode accordingly. It can be useful, but it’s usually best to think about what your subject and set S-AF or C-AF mode yourself. That way you can guarantee the camera will respond as you want it to.
Autofocus point selection
Auto AF Point selection systems get confused by busy backgrounds and fast-moving subjects. It’s far better to set the active AF point yourself. There’s usually an option that lets you choose a single point to use for focusing, this may be called something like Single AF-point or, ironically Multi AF Point (because you can choose from multiple points). Using this option ensures that the focus point is exactly where you want it to be, but it’s only effective if you can hold the camera so the active point is over the subject in the viewfinder and that’s often easier said than done when you’re shooting sport.
Zone AF (or similar) mode makes are great choice for shooting sport because it gives you a little bit of room for error when you’re trying to keep the active area over the subject, but it’s more focused than the Auto AF-Point which uses all the available points.
As with Single AF Point mode, in Zone AF mode you need to select the Zone that overlies the subject and then keep that area over the subject as it moves. Provided you do that and the camera is in C-AF mode, you should get sharp shots.