Canon EOS-1D X Mark III Advanced User's Manual page 391

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Movie Servo AF speed
You can set the Movie Servo AF's AF speed and its operation conditions. The function is
enabled when using a lens supporting slow focus transition during movie recording.*
When active
You can set [Always on] to have the AF speed take effect at all times for movie
recording (before and during movie recording) or set [During shooting] to have the AF
speed take effect only during movie recording.
AF speed
You can adjust the AF speed (focus transition speed) from the standard speed (0) to
slow (one of seven levels) or fast (one of two levels) to obtain the desired effect for the
movie creation.
* Lenses supporting slow focus transition during movie recording
USM and STM lenses released in and after 2009 are compatible. For details, refer to the
Canon website.
Caution
With certain lenses, even if you adjust the AF speed, the speed may not change.
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