Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 119)

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3 Selecting AI Servo AF Characteristics
Acceleration/deceleration tracking
This sets the tracking sensitivity for
moving subjects whose speed can
momentarily change dramatically by
starting or stopping suddenly, etc.
0
Suited for subjects that move at a steady speed (minor changes in
moving speed).
-2 / -1
Suited for subjects that move at a steady speed (minor changes 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 on 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 become unstable for
short periods.