Canon EOS-1D X Mark III Advanced User's Manual page 321

AI Servo AF/Servo AF for Moving Subjects
This AF operation is suited for moving subjects. While you hold down the shutter button
halfway, the camera will keep focusing on the subject continuously.
Viewfinder shooting (AI Servo)
• Once a subject is in focus, the focusing AF point is displayed. The focus indicator
< > in the viewfinder does not light up when subjects are in focus.
• The beeper will not sound even when focus is achieved.
• The exposure is set at the moment the picture is taken.
Live View shooting (Servo AF)
• Once a subject is in focus, the focusing AF point turns blue.
• The beeper will not sound even when focus is achieved.
• The exposure is set at the moment the picture is taken.
Caution
Depending on the lens used, the distance to the subject and the subject's speed,
the camera may not be able to achieve correct focus.
Zooming during continuous shooting may throw off the focus. Zoom first, then
recompose and shoot.
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