Canon EOS 600D - Manual (Page 255)

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3 Custom Function SettingsN
C.Fn III: Autofocus/Drive
C.Fn-7 AF-assist beam firing
The AF-assist beam can be emitted by the camera’s built-in flash or by an
external, EOS-dedicated Speedlite.
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 small flashes (like the built-in flash) from firing the AF-
assist beam.
With an EX-series Speedlite equipped with a LED light, the LED light will
not automatically turn on for AF-assist.
C.Fn-8 Mirror lockup
0: Disable
1: Enable
Prevents mechanical vibrations in the camera caused by the reflex
mirror action which can disturb shooting with super telephoto lenses or
close-up (macro) shooting. See page 122 for the mirror lockup
procedure.
If the external 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-7 is set to 0, 2, or 3.