Canon EOS-1D X Mark III DSLR Camera (Body Only) - 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.
375