Canon EOS 400D Digital Instruction Manual page 104

3 Setting Custom FunctionsN
C.Fn-3
0: Auto
The flash sync speed is set automatically within a range of 30 sec. to 1/
200 sec. to suit the scene's brightness.
1: 1/200sec. (fixed)
The flash picture can be taken without camera shake.
C.Fn-4
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 attain 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 focusing and exposure will always be at the
optimum point as you wait for the decisive moment.
C.Fn-5
You can enable or disable the camera's AF-assist beam or have it emitted
by the EOS-dedicated Speedlite instead.
0: Emits
1: Does not emit
The AF-assist beam is not emitted at all regardless of the shooting conditions. This
can prevent the AF-assist beam from disturbing another photographer's shooting.
2: Only external flash emits
With an EOS-dedicated Speedlite, it will emit the AF-assist beam when
necessary.
104
Flash sync. speed in Av (Aperture-priority AE) mode
Shutter/AE lock button
AF-assist beam