Canon EOS-1D X Instruction Manual page 91

Acceleration/deceleration tracking
[0]
Suited for subjects that move at a steady speed (minimal change in
moving speed).
[-2 / -1]
Suited for subjects that move at a steady speed (minimal change in
moving speed). It is effective when 0 is set but focus is unstable due
to the subject's slight movement or an obstacle in front of the subject.
[+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, which would result in a blurred subject. 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.
3 Selecting AI Servo AF Characteristics (For a Subject)
This sets the tracking sensitivity for
moving subjects whose speed can
suddenly change dramatically by starting
or stopping suddenly, etc.
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