Canon EF 600mm f/4L IS III USM EF600mm f/4L IS III USM Instruction 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. OO Autofocus will not resume even after you let go of an AF stop button in the following instances. ・Camera: EOS 630, EOS RT, EOS 5, EOS 10 In continuous shooting mode with [AI SERVO] ・Camera: EOS 5, EOS 10 In sports mode OO Useful when autofocus is operating mostly in [AI SERVO]. OO You can change the function of the AF stop buttons depending on the camera. Please refer to the camera's instructions for details. OO Angle positioning of the AF stop buttons is adjustable by a Canon Service Center at cost. ENG-10