Originally posted by pyrs n finns As a newbie, I read many different posts and replies in my quest for knowledge. So I just have to ask...when and why would you prefer to use the back button to focus rather than half press the shutter button?
First off, it takes a couple weeks of shooting before this becomes natural.
Essentially it gives you the opportunity to shut auto focus off without having to switch anything. On the weekend I was tracking an annoyingly fast little bird in bushes, the AF doesn't help trying to find it in the branches and brush. So I could press the shutter without it focusing or changing the focus that I set manually. A couple of times the bird went into the open and I was able to hit the AF button and get a shot.
AFS is handy for tough focus situations, and the AF button on the back gives you that. If you hold it down, you have AFC.