Back button focus....what did I ever do before I knew about you??? Before I knew about this great function, I struggled mightly with my focus on moving subjects. For example, a processional or recessional at a wedding. I had a hard time getting my subjects to focus while using the shutter button as my focus button. Well, those days are done thanks to this tip I learned, so thought I would share it with you here.
I shoot with the Canon 5D, and to set your camera:
1. under custom function #4 set it to #1: AE lock/AF (setting 0 is default)
2. turn your AF setting Al Servo and leave it there!
Normally, I try to put the focal point on the eye of my subject. However, with a moving subject I like to focus on the largest part of the body, with a high contrast area. For example, during processionals and recessionals I will focus on the man's torso area between his black tux and white shirt.
I love that the shutter button is no longer my "focus" button! And I love that when I am shooting a stationary subject, I can lock in the focus (push the * button with the focal point on the spot that I want in focus) and shoot away, even recomposing, and the focus will stay dead on.
Practice with this before you go out on a photo shoot! It took me a little bit to get used to, but now I am much more confident that my pictures are in focus!!
Hopefully someone out there found this helpful! :)
photo of the camera and tip from Melissa Jill