Back button focus is an incredibly useful tweak you can make on most modern dSLR camera bodies. For myself, it's applied on both my Canon EOS 5D Mark III and my 7D. On most cameras, it's available through any custom button mapping option(s) available.
What is it?
Back button focus (BBF) separates your shutter button and autofocus. At first, it seems awful. Why would you not want your shutter button to focus on your subject?
By separating the two functions, this allows you to focus on a single/set of AF points once and take a series of photographs without the shutter refocusing on the shot. This way, you're much more likely to capture many in-focus shots rather than a mixup of tack sharp shots and some that aren't in correct focus at all.
When using large apertures, this becomes incredibly useful. When shooting at a large aperture (ie., f/2.8 or lower), achieving sharp focus becomes increasingly difficult as your f-stop number goes down. Why? The field of focus becomes smaller and smaller. With an 85mm lens at f/1.8, your field of focus is a mere few inches. This just means that once you achieve focus, moving slightly will throw focus off. Your on-eye focus may become an on-nose focus instead. Rough!
Switching to BBF is a great thing. It takes a little while to become adapted to it, but any photographer can benefit from it.
Personally, I focus using my center AF point, recompose, then shoot.
I also enabled my shutter button to lock the exposure on a half-press. That way, I can "grab" a meter reading from an important part of my scene, lock the exposure, tap to focus, and complete the shutter cycle. From then on, I don't have to worry about refocusing if I'm shooting the same subject (for that single shot). I'd be able to keep shooting and have a series of usable shots, not just a bunch of missed-focus shots.
I was once shooting a boudoir session in Long Island, NY. I relied mostly on apertures ranging from f/1.4 up to f/2,8. BBF allowed me to capture a MUCH larger percentage of usable images even at such large apertures.
I strongly recommend all of you to give it a try. It's a tough change but it's worth all the effort.
-Aakaash Bali, Founder