Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 125)

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3 Customizing AF Functions
Enables or disables the EOS dedicated external 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. Useful when the
AF-assist beam may disturb others.
IR: IR AF assist beam only
When an external Speedlite is attached, only the infrared AF-assist
beam will be emitted. Useful when you do not want to fire a series of
small flashes as the AF-assist light (Intermittent flash method).
With an EX-series Speedlite equipped with an LED light, the LED
light will not automatically turn on as the AF-assist light.
AF-assist beam firing
If an external Speedlite’s [ AF-assist beam firing ] Custom Function is set to
[ Disable ], this function’s setting will be overridden and the AF-assist beam
will not be emitted.