Canon EOS-1D X Mark III Af Setting Manualbook page 36

Examples where the [-] setting is
effective.
Sports with far-away
subjects that experience
relatively minor changes
in speed, and
obstructions that often
pass in front of the
subject.
[Accel./decel tracking] option [-1/2] is a new parameter
for shooting in situations where subjects far-away have
minor changes in speed, and where the subject may be
obscured by obstructions in the foreground. This setting
is most effective for sports with wide playing fields, such
as soccer, or when you want to capture action on the far
side of the field. In many cases when photographing such
a scene, another athlete frequently moves across the
foreground obscuring the subject you are aiming at is
experiencing relatively minor changes in speed (the relative
speed changes become more significant when the shooting
distance is closer). In such a situation, [-1/2] is more effective
than the default setting of [0] for foreground obstructions.
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