Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 123)

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3 Customizing AF Functions
With AI Servo AF for continuous shooting, you can preset the AF
operation characteristics and shutter-release timing for all shots to be
taken after the first during continuous shooting.
0: Equal priority
Equal priority is given to focusing and
continuous shooting speed. In low
light or with low-contrast subjects,
shooting speed may slow down.
-2/-1: Shooting speed priority
Priority is given to the continuous shooting speed instead of achieving
focus. With -2, decrease of continuous shooting speed can be better
prevented than with -1.
+2/+1: Focus priority
Priority is given to achieving focus instead of the continuous shooting
speed. The picture will not be taken until focus is achieved. Although
focus can be achieved easier in low-light conditions with +2 than with
+1, continuous shooting speed may decrease.
AI Servo 2nd image priority
Under shooting conditions which activate flicker reduction (p.198), even if
[
Shooting speed priority ]: [ -1 ] or [ -2 ] is set, the continuous shooting speed
may become slower or the shooting interval may become irregular.
If focus cannot be achieved in low-light conditions when [ 0: Equal priority ]
is set, setting [ Focus priority ]: [ +1 ] or [ +2 ] may give better results.