Canon EOS 5D Mark III Instruction Manual (en)

 Customizing AF FunctionsN
Enables or disables the EOS-dedicated Speedlite’s AF-assist beam.
ON: Enable
The external Speedlite emits the AF-
assist beam when necessary.
OFF: Disable
The external Speedlite will not emit the 
AF-assist beam. This prevents the AF-
assist beam from disturbing others.
IR: IR AF assist beam only
Among external Speedlites, only models with an infrared AF-assist 
beam will be able to emit the beam. Set this if you do not want the AF 
assist to be emitted as small flashes.
You can set the AF operation characteristics and shutter-release timing 
for One-Shot AF.
: Focus priority
The picture is not taken until focus is 
achieved. Effective when you want to 
achieve focus before capturing the shot.
: Release priority
Priority is given to taking the picture 
instead of achieving focus. This gives 
priority to getting the shot rather than 
achieving correct focus.
Note that the picture will be taken 
even if focus has not been achieved.
AF-assist beam firing
One-Shot AF release priority
If the external Speedlite’s Custom Function [AF-assist beam firing] is set to 
[Disable], this function’s setting will be overridden and the AF-assist beam 
will not be emitted.