Canon EOS 40D Instruction Manual page 158

3 Custom Function SettingsN
C.Fn III -2
0: AF stop
1: AF start
AF operates only while the button is pressed. While the button is
pressed, AF operation with the camera is disabled.
2: AE lock
When the button is pressed, AE lock is applied. Convenient when you
want to focus and meter at different parts of the picture.
3: AF point: M 9 Auto/Auto 9 center
In the manual AF point selection mode, the button instantly switches to
automatic AF point selection only while you hold it down. Convenient
when you are no longer able to focus track a moving subject with a
manually-selected AF point in the AI Servo AF mode.
In the automatic AF point selection mode, the button selects the center
AF point only while you hold it down.
4: ONE SHOT z AI SERVO
In the One-Shot AF mode, the camera switches to AI Servo AF mode
only while you hold down the button. And in the AI Servo AF mode, the
camera switches to One-Shot AF mode only while you hold down the
button.
Convenient when you need to keep switching between One-Shot AF
and AI Servo AF for a subject which keeps moving and stopping.
5: IS start
With the lens' IS switch already <ON>, the Image Stabilizer operates
when you press the button.
The AF stop button is provided only on super telephoto IS lenses.
With setting 5, the Image Stabilizer will not operate when you press the
shutter button halfway.
158
Lens AF stop button function