Canon EOS-1DX Instruction Manual page 391

The ISO speed cannot be set for movie shooting.
If the shooting mode is <d/s/f/BULB>, the ISO speed will be set
automatically. In the <a> mode, you can freely set the ISO speed
(p.232).
For movie shooting, ISO 32000/40000/51200 cannot be set.
Under [z2: ISO speed settings], if [ISO speed range]'s [Maximum]
is set to [51200/H] or higher, the manual setting range's Maximum will
be expanded to enable ISO 32000/40000/51200 to be set. However,
since movie shooting at ISO 32000/40000/51200 may result in
substantial noise, the expanded ISO speed ("H" displayed) will be
used.
The manually set ISO speed changes when switching to
movie shooting.
If you shoot a movie when [Maximum: 51200] is set with [ISO speed
range] and ISO speed is set to ISO 32000/40000/51200, the ISO
speed will switch to ISO 25600 (during movie shooting with manual
exposure). Even if you switch back to still photo shooting, the ISO
speed will not revert to the original setting.
If you shoot a movie when L (ISO 50) is set, the ISO speed setting will
switch to ISO 100 (during movie shooting with manual exposure).
Even if you switch back to still photo shooting, the ISO speed will not
revert to the original setting.
The exposure changes during movie shooting.
If you change the shutter speed or aperture during movie shooting, the
changes in the exposure may be recorded.
Zooming the lens during movie shooting can cause changes in the
exposure regardless of whether the lens' maximum aperture changes
or not. Changes in the exposure may be recorded as a result.
Troubleshooting Guide
391