Back-button autofocus will change your photography for the better. Pete Wilson-Jones explains.
If you have never used back-button autofocus you're missing out on a very useful feature. It only takes a few minutes to master and it will almost certainly improve your photography.
Shutter half pressed: If you're like most photographers your routine when taking a photo is to half-press the shutter button to activate the camera's autofocus and metering, then fully press the button to take the shot. That works pretty well most of the time but it’s far from perfect. For one, because the focus and exposure functions are co-located on the same button, it’s not possible to focus on one part of the scene while metering on another. There's another issue. The “half-press” method is quite slow, and that causes focus problems when you're shooting moving subjects. The trouble is the lag that occurs between setting the focus and firing the shutter. If the subject is moving fast enough it will be out of focus by the time the shutter fires.
Back-button autofocus: When you think about it, separating the autofocus and exposure functions makes a lot of sense. Which is why some DSLRs offer an independent focus activation button, known variously as “back-button autofocus”, “autofocus lock”, AF-L or AF-ON.
Generally located on the back of the camera, the button lets you set and lock the focus with your thumb – independent of what’s happening with the shutter-release button.
On most pro and semi-pro cameras like the Canon EOS 7D (left) and Nikon
D5000 (right), back-button autofocus (circled) is active by default. With some
consumer DSLRs, like the EOS 450D, back-button autofocus has to be
enabled in the camera’s menu.
Action photography: Back-button focusing is particularly good for sport and action photography. Being able to prefocus separately eliminates the lag when the shutter is pressed, enabling more precise timing, and a more accurate exposure if you happen to recompose the image after back-button focusing (using the shutter release to lock focus also locks the exposure at the same time). Further, if you are focusing on a subject in the distance and someone crosses in the foreground, it’s easy to hold the focus on the original player without worrying about the focus tracking back and forth.
Portrait photography: While timing and speed aren’t big issues for portrait photography, back-button focusing makes it quick and easy to focus on a picture element that is not in the middle of the photo – focus, press the back-button focus button and recompose. And because the exposure and focus points can be set separately, you have more flexibility to cope with high-contrast lighting situations.
Which cameras: Unfortunately not all cameras offer back-button autofocus. It’s common on pro and semi-pro DSLRs, less so on consumer DSLRs. Check your manual if you’re not sure. With some cameras, like the Canon EOS 450D, it is included as a Custom Function option which has to be set up in the camera menu.
If you do have a camera with back-button autofocus give it a try. It’s doesn’t take long to get used to and you’ll soon find yourself wondering how you managed without it!
Article first published in Digital Photography + Design, October-November 2011.
Locking the focus with the back-button freed up the shutter release so I
could take photos without worrying about the lens refocussing on the
net or the player in the foreground. With back-button focusing, correct
exposure is determined at the moment each shot is taken. (Canon 7D,
70-200mm lens @ 200mm, 1/2000 sec @f7.1, ISO 400, monopod.)
Back-button autofocus is particularly useful when the subject is off
centre. In this penalty shot I used the AF-ON button to focus on the
ball then recomposed to include the goals and the other players. When
the player took the kick I fired the shutter – with no noticeable
shutter lag. (Canon 7D, 70-200mm lens @ 76mm, 1/800 sec @f8, ISO 400, monopod.)