Canon RF 400mm f/2.8 L IS USM Instructions Manual page 11

5. AF Stop Buttons
You can use the AF stop buttons to temporarily
pause autofocus.
AF stop buttons
During autofocus operation, you can press an
AF stop button to temporarily pause autofocus,
and then release the button to resume.
Press an AF stop button to maintain a focusing
distance or to avoid focus search.
Press the shutter button while holding down
an AF stop button to shoot at that focusing
distance.
Useful when autofocus is operating mostly in
O O
[SERVO AF].
You can change the function of the AF stop
O O
buttons depending on the camera. Please refer
to the camera's instructions for details.
Angle positioning of the AF stop buttons is
O O
adjustable by a Canon Service Center at cost.
ENG-10