Canon EOS 6D Instruction Manual page 308

3 Custom Function SettingsN
C.Fn II -2
[+2/+1]
Effective for subjects having sudden movements, sudden acceleration/
deceleration, or sudden stops. Even if the moving subject's speed suddenly
changes dramatically, the camera continues to focus the target subject. For
example, for an approaching subject, the camera becomes less prone to
focus behind it to avoid subject blur.
For a subject stopping suddenly, the camera becomes less prone to focus in
front of it. Setting +2 can track dramatic changes in the moving subject's
speed better than with +1.
However, since the camera will be sensitive to even slight movements of the
subject, the focusing may be unstable momentarily.
C.Fn II -3
[Equal priority]
Equal priority is given to focusing and shutter release.
[Release priority] (
Pressing the shutter button takes the picture immediately even if focus has not
been achieved. It is effective when you want give priority to the shutter chance
rather than to 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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Acceleration/deceleration tracking
This sets the tracking sensitivity for moving
subjects whose speed can suddenly change
dramatically by starting or stopping suddenly, etc.
[0]
Suited for subjects that move at a fixed speed.
AI Servo 1st image priority
You can set the AF operation characteristics and
shutter-release timing for the first shot during
continuous shooting with AI Servo AF.
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