Canon EOS 5D Mark III Instruction Manual page 104

3 Customizing AF FunctionsN
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.
VF display illumination
The AF points and grid in the viewfinder can be illuminated in red when
focus is achieved.
The setting here is not applied to the electronic level display (p.59) in the
viewfinder.
When you press the <S> button, the AF points and grid will be illuminated
in red regardless of this setting.
AF Microadjustment
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 105.
104
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.