Canon EOS-1D X Mark III Advanced User's Manual page 266

Caution
ISO speed cannot be expanded to L, H2, or H3 (equivalent to ISO 50, ISO 409600,
and ISO 819200, respectively) in movie recording.
During movie recording, avoid changing the shutter speed, aperture value, or ISO
speed, which may record changes in the exposure or create more noise at high
ISO speeds.
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.
Note
Exposure compensation with ISO Auto can be set in a range of ±3 stops when [
6: Custom Controls] is set to [
When ISO Auto is set, you can press the <
locking the ISO speed during movie recording, you can cancel it by pressing the
< > button. (ISO speed lock is maintained until you press the <
If you press the <
level difference on the exposure level indicator (
button was pressed.
: Expo comp (hold btn, turn
> button and recompose the shot, you can see the exposure
266
> button to lock the ISO speed. After
) compared to when the <
)] ( ).
> button.)
>