Have heard about this before but didnt take the time to try and understand it. Decided to research it a tad before experimenting and perhaps losing a decent shot :-(
The advantages of using this function.
Back-button AF is engaged by setting the appropriate Custom Function in your EOS camera. Custom Functions are totally locked-out if you’re in the full-auto or a picture-icon setting.
The particular Custom Function number varies, depending on the EOS model in question.
EOS Rebel T3: C.Fn 7 (option 1 or 3)
EOS Rebel T3i: C.Fn 9 (option 1 or 3)
EOS Rebel T4i: C.Fn 6 (option 1 or 3)
EOS 60D: C.Fn IV-1 (option 1, 2, 3, or 4)
EOS 7D: C.Fn IV-1 (Custom Controls — Shutter, AF-ON, AEL buttons)
EOS 6D: C.Fn III-5 (Custom Controls -- Shutter, AF-ON, AEL buttons)
EOS 5D Mark II: C.Fn IV-1 (option 2 or 3)
EOS 5D Mark III: C.Fn menu screen 2 (Custom Controls -- Shutter,