Canon EOS REBEL T4I Instruction Manual page 100

Custom Function Settings*
¢.Fn IIh Autofocus/Drive
The settings to specify whether or not the AF-assist beam is emitted by the
camera's built-in flash or by an external, EOS-dedicated Speedlite can be
set.
0: Enable
If necessary, the AF-assist beam will be emitted by the built-in flash or
external Speedlite.
1 : Disable
The AF-assist beam is not emitted.
2: Enable external flash only
If an external Speedlite is attached, it will emit the AF-assist beam when
necessary. The camera's built-in flash will not fire the AF-assist beam.
3: IR AF assist beam only
Among external Speedlites, only those which have an infrared AF-assist
beam will be able to emit the beam. This prevents any Speedlite which
uses a series of intermittent flashes (like the built-in flash) from firing the
AF-assist beam.
With an EX-series Speedlite equipped with an LED light, the LED light
will not automatically turn on for AF-assist.
[] If the external Speedlite's [AF-aSsist beam firing] Custom Function is Set
t° [Disabled], the Speed!ite wil! not emi t the AF-assist beam eve n if the
camera's C,Fn-4- is setto 0,2, or3,
0: Disable
1 : Enable
Prevents mechanical vibrations in the camera caused by the reflex
mirror action (mirror shock), which can disturb shooting with super
telephoto lenses or close-up (macro) shooting. See page 140 for the
mirror Iockup procedure.
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