I've heard some advanced photographers talk about using back-button autofocus. As I understand it, this involves changing one of the camera's custom setting to engage the AF when a button on the rear of the camera is pressed (as opposed to half-pressing the shutter button).
What is the advantage to back-button autofocus? Any suggestions on why one would want to switch to this method?
Many years ago I tried splitting focus from the shutter because I was shooting action-sports. It took about a day to decide I liked it.
One of the things we're supposed to do when we're shooting is keep both eyes open; That helps avoid fatigue from shooting for hours, but also lets us see what is going on around us. That is smart in case good action is happening to the side. It's also good because you might need to be aware of an unsafe condition unfolding while you're shooting.
Anyway, after many years of using that sample configuration on my bodies, one of the things I enjoyed again and again was the ability to track some action, see someone or something about to get in the way, release the focus button so it wouldn't mis-track, then press it again as I panned past the intervening object. It was REALLY useful.
Otherwise, being able to tap the focus button to pre-focus was great. It also worked really well when using a tripod to shoot stills or landscapes, because I could focus then release the shutter without the camera trying to refocus.
Sports Illustrated has a site for their photographers that defines the camera settings they recommend for pro-am and pro bodies. Their settings specify what custom settings need to be set to enable the split functions. The specs are a bit old, but should be good enough to get you there.
As Cabbey said, it's to seperate engaging Auto-focus from actually taking the picture.
Setting the focus, and taking the picture should be two distinct steps, and this configuration enforces that mentality.
It's advantages become even more apparent when doing "Focus/Recompose." With a standard configuration, it's too easy to re-focus accidentally, and with a dedicated button, it's much harder.
The main reason I have used it is to separate AF from AE. Normally the shutter release button does both, but sometimes you just don't want that. So you move one (or the other, depending on what your camera is capable of) from the shutter release (normally 1/2 depressed will trigger it) to a back button, ideally one that's easy to get to, like under your thumb.
If you want the camera to always engage the autofocus every time you press the shutter button, by all means leave AF coupled to the shutter and think no more of it. If, on the other hand, you want a bit more control of precisely when and where the camera focuses, if at all (USM lenses with full-time manual focus override come to mind, especially macro ones), then decoupling it and using the separate AF button is the way to go. I did so years ago and now get slightly desperate whenever I am handed a camera where AF and shutter is still coupled!
The perks for decoupling autofocus from the shutter include prefocusing, selective focusing with moving action, priming the IS system, and recomposing. It helps me think/experiment with focus and composition more. Also, on Pentax, it lets me basically enable full time manual focus by pushing in the lens release button while in AF mode.
I tried doing it for about 2 days and found that with heavier lenses on my camera I couldn't grip the camera as well having lost my thumb to the additional task of focusing. I have reverted to the traditional way of doing it.
Best bet... give it a go for a day or 2. If you like it, brilliant, if you don't you don't :) I don't think it offers a distinct advantage, it is more of a preference than anything in my opinion.
I use the back button on my Canon 5DII in AF Servo mode. That way, I get the benefits of predictive autofocus when I need it (with moving subjects, for example), but the autofocus stops as soon as I release the button, so it's also somewhat like the "OneShot" autofocus mode.
I now have full control over metering, focus and the moment of exposure because they are on three different buttons instead of being mashed into a single one. Why should I let the camera second-guess me? Getting used to this is a matter of minutes.
There is one caveat to this. Many battery grips (my BG-E2N included) don't have the button that does this and therefore if you do a lot of battery grip portrait shots, this setting isn't really an option.
