Canon EOS 5D MARK III Instruction Manual page 94

Customizing AF Functions _
With the [AF2] to [AFS] menu tabs, you
can set the AF functions to suit your
shooting style or subject.
AF2: AI Servo
You can set the AF operation characteristics and shutter-release timing
for the first shot during continuous shooting with AI Servo AF.
D/®: Equal priority
Equal priority is given to focusing and
shutter release.
D: Release priority
Pressing the shutter button takes the
picture immediately even if focus has not
been achieved. This gives priority to
getting the shot rather than achieving
correct focus.
®: Focus priority
Pressing the shutter button does not take the picture until focus has
been achieved. Effective when you want to achieve focus before
capturing the shot.
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