Canon EOS Rebel T4i 18-135mm IS STM Lens Kit Instruction Manual page 197

Cautions when [Movie servo AF] is set to [Enable]
Shooting conditions that make focusing difficult
A fast-moving subject approaching or moving away from the camera.
A subject moving at a close distance in front of the camera.
Also refer to "Shooting conditions that make focusing difficult" on page
159.
• Since using Movie Servo AF will consume battery power, the number of
possible shots and the possible movie shooting time will be reduced.
• During zooming or image magnification, Movie Servo AF operation will be
stopped momentarily.
• During movie shooting, if a subject approaches / moves away, or if
the camera is moved vertically or horizontally (panning) the recorded
movie image may momentarily expand or contract (change in image
magnification).
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