Canon EOS 5D Mark III Instruction Manual (en)

Using AF to Focus
The dedicated AF sensor is used to focus in One-Shot AF mode (p.70), 
using the same AF method as with viewfinder shooting.
Although you can focus the target area quickly, the Live View image 
will be interrupted momentarily during the AF operation.
In AF area selection modes other than 61-point automatic selection AF, 
you can manually select the AF point or zone. In the <
A> mode, “61-
point automatic selection AF” is set automatically.
Display the Live View image.
Press the <0> button.
The Live View image will appear on 
the LCD monitor.
If the AF area selection mode is set to 
“61-point automatic selection AF”, the 
Area AF frame will be displayed.
In other modes, the AF point will be 
displayed as a small frame.
The larger rectangular frame is the 
magnifying frame.
Select the AF area selection 
Press the <S> button.
Each time you press the <B> 
button, the AF area selection mode 
Quick Mode: f
Area AF frame
Magnifying frame