Canon EOS Rebel T5i 18-55mm IS STM Lens Kit - Manual (Page 99)

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f: Changing the Autofocus OperationN
AI Focus AF switches the AF operation from One-Shot AF to AI
Servo AF automatically if a still subject starts moving.
After the subject is focused in One-Shot AF operation, if the subject
starts moving, the camera will detect the movement, change the AF
operation automatically to AI Servo AF, and keep tracking the
moving subject.
AI Focus AF for Automatic Switching of AF Operation
When focus is achieved with AI Focus AF with the Servo operation active,
the beeper will continue beeping softly. However, the focus confirmation
light in the viewfinder will not light up. Note that focus will not be
locked in this case.
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