Canon EOS 5DS Instruction Manual page 115

Acceleration/deceleration tracking
0
Suited for subjects that move at a steady speed (minimal change in
moving speed).
+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 even to the slight
movements of the subject, the focusing may momentarily become
unstable.
3 Selecting AI Servo AF CharacteristicsN
This sets the tracking sensitivity for
moving subjects whose speed can
suddenly change dramatically by starting
or stopping suddenly, etc.
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