Canon EOS Rebel T3 18-55mm IS II Kit Instruction Manual page 221

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.
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-6 is set to 0, 2, or 3.
C.Fn IV: Operation/Others
C.Fn-7
0: AF/AE lock
1: AE lock/AF
Convenient when you want to focus and meter separately. Press the
<A> button to autofocus, and press the shutter button halfway to apply
AE lock.
2: AF/AF lock, no AE lock
In the AI Servo AF mode, you can press the <A> button to stop the AF
operation momentarily. This prevents the AF from being thrown off by
any obstacle passing between the camera and subject. The exposure is
set at the moment the picture is taken.
3: AE/AF, no AE lock
This is useful for subjects which keep moving and stopping repeatedly.
In the AI Servo AF mode, you can press the <A> button to start or stop
the AI Servo AF operation. The exposure is set at the moment the
picture is taken. Thus, the optimum focusing and exposure will always
be achieved as you wait for the decisive moment.
Shutter/AE lock button
3 Custom Function SettingsN
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