Canon EOS-1D X Mark III Advanced User's Manual page 259

Caution
Avoid adjusting shutter speed while recording movies, which will record changes in
exposure.
When recording a movie of a moving subject, a shutter speed of approx. 1/25 sec.
to 1/125 sec. is recommended. The faster the shutter speed, the less smooth the
subject's movement will look.
If you change the shutter speed while recording under fluorescent or LED lighting,
image flicker may be recorded.
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