Canon EOS-1D X Mark III Advanced User's Manual page 390

Caution
When set to [Movie Servo AF: 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 to the camera.
• When shooting with a higher aperture value.
• Also see Shooting Conditions That Make Focusing Difficult
area selection modes (for viewfinder shooting).
Since the lens is driven continuously and the battery power is consumed, the
possible movie recording time (
If AF operations are performed or you control the camera or lens during movie
recording, the camera's built-in microphone may also record mechanical sounds of
the lens or sounds of camera/lens operations. In this case, using an external
microphone may reduce these sounds. If the sounds are still distracting with an
external microphone, it may be more effective to remove the external microphone
from the camera and position it away from the camera and lens.
Movie Servo AF will pause during zooming or magnified view.
During movie recording, if a subject approaches or 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).
) will be shortened.
390
in the section on AF