Canon EOS 90D, Instruction Manual, Page: 771 / 888

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Custom Function Setting Items
C.Fn II: Autofocus
C.Fn II-1: Tracking sensitivity
You can set the subject-tracking sensitivity, which aff ects responsiveness to
interfering objects that move across AF points or subjects that stray from AF
points during AI Servo AF/Servo AF.

0
Standard setting. Suitable for moving subjects in general.

Locked on: –2 / Locked on: –1
The camera will try to continue focusing on the subject even if an obstacle
cuts across the AF points or if the subject strays from the AF points. The
–2 setting makes the camera keep tracking the target subject longer than
the –1 setting.
However, if the camera focuses on a wrong subject, it may take slightly
longer to switch and focus on the target subject.

Responsive: +2 / Responsive: +1
The camera can focus consecutively on subjects at diff erent distances
that are covered by the AF points. Also eff ective when you want to always
focus on the closest subject. The +2 setting is more responsive than the
+1 setting when focusing on the next subject.
However, the camera will be more prone to focus on an unintended
subject.