Canon EOC-1DX Instruction Manual page 94

3 Customizing AF Functions
AF-assist beam firing
Enables or disables the EOS-dedicated Speedlite's AF-assist beam.
IR: IR AF assist beam only
Only external Speedlite models that have an infrared AF-assist beam
will emit the AF-assist beam. Set this if you do not want the AF assist
using small flashes to be emitted.
If the external Speedlite's Custom Function [AF-assist beam firing] is set to
[Disable], the AF-assist beam will not be emitted, regardless of this
function's setting.
One-Shot AF release priority
You can set the AF operation characteristics and shutter-release timing
for One-Shot AF.
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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.
: 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.
Note that the picture will be taken
even if focus has not been achieved.