Canon EOS Rebel T1i Instruction Manual page 187

C.Fn III: Autofocus/Drive
C.Fn-8
The AF-assist beam can be emitted by the camera's built-in flash or by the
external, EOS-dedicated Speedlite.
0: Enable
1: Disable
The AF-assist beam is not emitted.
2: Only external flash emits
If an external, EOS-dedicated 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.
If the external, EOS-dedicated Speedlite's [AF-assist beam firing] Custom
Function is set to [Disabled], the Speedlite will not emit the AF-assist beam
even if the camera's C.Fn-8-0/2 is set.
C.Fn-9
0: Disable
1: Enable
Prevents camera vibrations caused by the reflex mirror action which can
disturb shooting with super telephoto lenses or close-up (macro)
shooting. See page 104 for the mirror lockup procedure.
AF-assist beam firing
Mirror lockup
3 Custom Function SettingsN
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