Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 367)

Accel./decel. tracking
Setting for AI Servo AF/Servo AF subject-tracking sensitivity in response to sudden,
significant changes in speed, as when subjects suddenly start or stop moving.
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).
Effective when a setting of 0 makes focus unstable, due to slight subject movement or
an obstruction 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 significantly, the
camera continues to focus on the target subject. For example, the camera is less likely
to focus behind a subject that suddenly starts approaching you, or in front of an
approaching subject that suddenly stops moving. 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,
focusing may become unstable for short periods.
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