Canon EOS 5D Mark III Instruction Manual (en)

 Customizing AF FunctionsN
: Manual select.:Zone AF
The 61 AF points are divided into nine zones for focusing.
: Auto selection: 61 pt AF
All of the AF points are used for focusing.
You can set the method for changing the AF area selection mode.
 M-Fn button
After you press the <S> button, each 
time you press the <B> button, the 
AF area selection mode changes.
 Main Dial
After you press the <S> button, turning 
the <6> dial changes the AF area 
selection mode.
AF area selection method
The <X> mark cannot be deleted from [Manual selection:1 pt AF].
If the attached lens belongs to group H (p.84, 85), you can only select 
[Manual select.:Spot AF], [Manual selection:1 pt AF], and [Expand 
AF area: 
When [  
 Main Dial] is set, use the <9> to move the AF point