Canon EOS 90D Instruction Manual page 772

Custom Function Setting Items
C.Fn II-2: Acceleration/deceleration tracking
Sets the AI Servo AF/Servo AF tracking sensitivity for subjects that change
speed suddenly, as by suddenly moving or stopping.
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). Eff ective when a setting of 0 makes focus unstable, due
to slight subject movement or an obstruction in front of the subject.
+2 / +1

Eff ective 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, 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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