Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 90)

f : Selecting the AF Operation
90
This AF operation is suited for
moving subjects when the focusing
distance keeps changing. While you
hold down the shutter button halfway,
the camera will keep focusing on the
subject continuously.
The exposure is set at the moment the picture is taken.
When the AF area selection mode (p.96) is set to Automatic
selection AF, focus tracking will continue as long as the Area AF
frame covers the subject.
When you press the shutter button
halfway and the camera is focusing with
AF, the <
i > icon will appear on the
lower right of the viewfinder.
In One-Shot AF mode, the icon also
appears if you press the shutter button
halfway after focus is achieved.
AI Servo AF for Moving Subjects
AF Operation Indicator
With AI Servo AF, the beeper will not sound even when focus is achieved.
Also, the focus indicator <
o > in the viewfinder will not light up.
The AF operation indicator can be displayed outside the viewfinder’s image
area (p.137).