Canon eos rebel t1i - Manual (Page 187)

187
3 Custom Function SettingsN
C.Fn III: Autofocus/Drive
C.Fn-8 AF-assist beam firing
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.
C.Fn-9 Mirror lockup
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.
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.