Canon EOS 5D Mark III Instruction Manual (en)

 Customizing AF FunctionsN
The AF points and grid in the viewfinder can be illuminated in red when 
focus is achieved.
AUTO: Auto
The AF points and grid are automatically 
illuminated in red under low light.
ON: Enable
The AF points and grid are illuminated in 
red regardless of the ambient light level.
OFF: Disable
The AF points and grid are not 
illuminated in red.
You can make fine adjustments for the AF’s point of focus. For details, 
see “Fine Adjustment of AF’s Point of Focus” on page 106.
VF display illumination
If [Selected (pre-AF, focused)] or [Selected (focused)] is set, the AF point 
will not be displayed even when focus is achieved with AI Servo AF.
AF Microadjustment
The setting here is not applied to the electronic level display (p.59) in the 
When you press the <S> button, the AF points and grid will be illuminated 
in red regardless of this setting.