Canon EOS-1D X Mark III Advanced User's Manual page 375

AF-assist beam firing
You can enable or disable the AF-assist beam of a Speedlite for EOS cameras.
[ ] Enable
Enables firing of the AF-assist beam, when needed.
[ ] Disable
Disables firing of the AF-assist beam. Set if you prefer not to fire the AF-assist beam.
[ ] IR AF assist beam only
Enables infrared AF-assist beam firing by external Speedlites equipped with this feature,
when these flash units are attached.
Caution
If a Speedlite's [AF-assist beam firing] Custom Function is set to [Disable], the
AF-assist beam will not be emitted.
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