Canon EOS 5D Mark III Instruction Manual (en)

 Selecting AI Servo AF Characteristics (For a Subject)N
AF pt auto switching
This sets the switching sensitivity of the 
AF points as they track the subject 
moving dramatically up, down, left, or 
This setting takes effect in the following 
AF area selection modes: AF point 
expansion (Manual selection 
), AF 
point expansion (Manual selection, 
surrounding points), Zone AF (Manual 
selection), 61-point automatic selection 
Standard setting for gradual AF point switching.
[+2 / +1]
Even if the target subject moves dramatically up, down, left, or right 
and moves away from the AF point, the AF point switches to another 
one to continue focusing the subject. The camera switches to the AF 
point deemed most likely to focus the subject based on the subject’s 
continual movement, contrast, etc. Setting +2 makes the camera 
more prone to switch the AF point than with +1.
However, with a wide-angle lens having a wide depth of field or if the 
subject is too small in the frame, the camera may focus with the 
wrong AF point.