Canon EOS-1D Mark IV - Manual (Page 220)

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 -♦ Auto/Auto -► 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 Al 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 Ti Al SERVO
In the One-Shot AF mode, the camera switches to Al Servo AF mode
only while you hold down the button. And in the Al 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 Al 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 <FEL> 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.
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