Canon EOS-1D X Mark III Advanced User's Manual page 273

Caution
The card's writing/reading speeds required for recording movies (card
performance requirements) vary depending on the movie recording quality.
Before recording movies, see
performance requirements.
Movie Recording Size
Image size
*
* Movies can be cropped around the center of the image, as if recorded with a telephoto lens.
Caution
If you change the [ : Video system] setting, also set [
again.
Other devices may not play movies such as 4K,
Rate movies normally, because playback is processing-intensive.
Apparent resolution and noise vary slightly depending on movie recording quality
and settings for the lens used.
Note
To obtain better performance with the card, formatting the card with the camera
before recording movies is recommended (
Movies cannot be recorded in HD or VGA quality.
Cards that can record movies
Image Size
5472×2886
4096×2160
4096×2160
3840×2160
1920×1080
273
to check card
: Movie rec quality]
/ , and High Frame
).
Aspect Ratio
17:9
17:9
17:9
16:9
16:9