Canon EOS-1D X Mark III Advanced User's Manual page 261

3.
Set the aperture value (1).
Turn the <
4.
Focus and record the movie.
The procedure is the same as steps 3 and 4 for
Caution
Avoid adjusting the aperture value while recording movies, which will record
changes in exposure caused by aperture adjustment.
> dial to set it.
261
Program AE.