Canon EOS-1DS MARK III Instruction Manual page 60

Custom Function Settings
1: AF priority/Drive speed priority
For the first shot, focusing the subject is given priority. During continuous
shooting, the continuous shooting speed is given priority over the focus°
tracking of the subject.
2: Release/Drive speed priority
For the first shot, shutter release is given priority over focusing the
subject. During continuous shooting, the continuous shooting speed is
given priority more than with setting 1.
In the AI Servo AF mode while you are focusotracking a subject, the camera
can either continue focusing the target subject even if a closer subject
(closer than at the main focus point) suddenly appears in the picture, or the
camera can switch to focus the closer subject.
*Main focus point = With automatic AF point selection: Center AF point
With manual AF point selection + AF point expansion
(C.Fn IH -8ol/2): Manually-selected AF point
0: Main focus point priority
The active AF point witl switch to the main AF point and start focusing
the closer subject. Convenient when you always want to focus the
closest subject.
1 : Continuous AF track priority
Any closer subject appearing in the picture will be ignored as an
obstruction. The main focus point does not take priority, so the tracking
of the target subject can continue and switch to an adjacent AF point
based on the preceding focusing result. Convenient when obstacles
such as telephone poles go in front of the target subject.
If autofocus is executed, but focus cannot be achieved, the camera can
either keep trying to focus or stop.
0: Focus search on
1: Focus search off
Prevents the camera #om becoming grossly out of focus as it attempts
to focus again. Especially convenient with super telephoto lenses which
can become extremely out of focus.
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