I seem to be doing the opposite of everyone else on this thread (which probably means I'm missing something).
I used back button focus on my t2i/550d for everything. I would use the center point, and the focus and recompose. Like most others, this worked pretty well for me, except for issues with nobody else being able to use my camera, and occasional missed focus (Either due to inaccuracies in the body itself, or too much movement during the recompose).
Once I upgraded to a 5d3, I actually went the other way and have stopped using back button focus, with the following rational:
- I don't shoot sports/birds/wildlife, and so am more interested in accurate focus than I am worried about missing a shot. With back button focus, the shutter will release regardless of if focus has been acquired or not. To that end I have set AI-Servo 1s5/2nd Image Priority to prioritize focus over shutter release.
- The spread of AF points on the 5d3 is (usually) wide enough where I don't need to focus and recompose, I can just pick an AF point on the outside and am able to frame the shot as I would like.
Having said that, I do run into times when I can't get an AF point where I need it, and I feel like I was faster at focus and recompose than I am at selecting a different AF point, but perhaps this is just a practice/technique thing.
I would love to hear from others on this, with so many others in this thread using back button focus on 5d3/1dx I feel like I must be missing something.