Focus is usually achieved by half-pressing the shutter-release button, but it’s also possible to customise most SLRs so that a button on the back of the camera – usually marked AF-ON or * – activates autofocus instead. This function, often used by professional photographers, is called ‘back button focus’.
Back button focus is useful when you’re using continuous autofocus, or focus tracking. Using the shutter button to focus is fine for static subjects, but with moving subjects it can be hard to tell whether or not the shutter is half-pressed, and if you accidentally press too hard, you run the risk of firing off a burst of shots too early.
There’s also a risk that when you come to fully press the shutter button, you’ll inadvertently lift your finger off the button for a split second before fully pressing it. In this split second, focus tracking will stop, and if the subject moves closer to the camera, the shot will be blurred.
The advantage of so-called back-button focus is that you don’t need to half-press anything – you just hold down the back AF button with your thumb to start focus tracking, keep holding it down as you track your subject’s movement, and then fire off a burst at the peak of the action – still with the back button fully-pressed.
Not all cameras have an AF button on the back, but on most models back-button focus can be set using a Custom Function.