Canon EOS REBEL T7i Instruction Manual page 396

3 Custom Function Setting ItemsN
C.Fn-12
0: AF/AE lock
1: AE lock/AF
This is 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
During AI Servo AF (or Servo AF for Live View shooting), you can
press the <A> 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 (or Servo AF for Live View
shooting), 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, you can set your camera to always maintain the
optimum focusing and exposure and wait for the decisive moment.
During movie shooting
With setting 1 or 3, press the <A> button for One-Shot AF.
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With setting 2, press the shutter button halfway for One-Shot AF.
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396
Shutter/AE lock button