Canon MARK IV User Manual (en)

Custom Function Settings
C.Fn III -6     Lens AF stop button function
* The AF stop button is provided only on super telephoto IS lenses.
0: AF stop
1: AF start
AF operates only while the button is pressed. While you hold down the 
button, AF will not work on the camera.
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 (among 45 AF points) 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.
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 
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 Image Stabilizer will not operate when 
you press the shutter button halfway.
6: Switch to registered AF point
While holding down the AF Stop button, press the <I> button to 
switch to the registered AF point. Press it again to switch to the previous 
AF point. To register the AF point, see page 224.
7: Spot AF
The focusing line sensor’s active area is made narrower to enable a 
narrower part to be focused. This works in all AF modes and with any AF 
point selection method. It is especially convenient during manual AF 
point selection. However, since it is difficult to keep the AF point on a 
moving subject, focusing might be difficult. With spot AF, the AF point 
will blink brighter than usual.