Canon EOS-1 DX Mark III Advanced User's Manual page 278

Cards That Can Record Movies
For details on cards that can record at each level of movie recording quality, check the
Canon website.
Test cards by recording a few movies to make sure they can record correctly at your
specified size ( ).
Caution
Before recording 4K movies, format cards by selecting [Low level format] in [ :
Format card] (
If you use a slow-writing card when recording movies, the movie may not be
recorded properly. Also, if you play back a movie on a card with a slow reading
speed, the movie may not be played back properly.
When recording movies, use high-performance cards with a writing speed
sufficiently higher than the bit rate.
When movies cannot be recorded properly, format the card and try again. If
formatting the card does not resolve the problem, refer to the card manufacturer's
website, etc.
Note
To obtain better performance with the card, formatting the card with the camera
before recording movies is recommended (
).
).
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