Canon REBEL SL1 EOS 100D Instruction Manual page 7

Custom Function Settings*
Enables or disables the built-in flash's AF-assist beam or the EOS-
dedicated external Speedlite's AF-assist beam.
0: Enable
The AF-assist beam will be emitted when necessary.
1 : Disable
The AF-assist beam is not emitted. This prevents the AF-assist
beam from disturbing others.
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
When an external Speedlite is attached, only with infrared AF-
assist beam will be emitted. This prevents the AF-assist beam
from firing as a burst of small flashes.
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
to [Disable], this function's setting will be overridden and the AF-assist
beam wilt not be emitted.
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 147 for
the mirror Iockup procedure.
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