Canon EOS 1300D - Manual (Page 265)

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3 Custom Function SettingsN
C.Fn IV: Operation/Others
C.Fn-8 Shutter/AE lock button
0: AF/AE lock
1: AE lock/AF
This is convenient when you want to focus and meter separately.
Press the button to autofocus, and press the shutter button
halfway to apply AE lock.
2: AF/AF lock, no AE lock
During AI Servo AF, you can press the button to pause 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 that keep moving and stopping
repeatedly. During AI Servo AF, you can press the button to
start or stop the AI Servo AF operation. The exposure is set at the
moment the picture is taken. Thus, you can set your camera, so
that the optimum focusing and exposure can be achieved and wait
for the decisive moment.
During Live View shooting
 With setting 1 or 3, press the button for One-Shot AF.
 With setting 0 or 2, press the shutter button halfway for One-Shot AF.