Canon EOS-1D X Instruction Manual page 32

k Shooting Movies
If [z5: Movie shoot. btn] is set to [V/o], still photo shooting is not
possible.
Since the image circle of EF Cinema zoom lenses does not support the
image size of 35mm full-frame size image sensor, still photo shooting is
not possible. (The image periphery will look dark.)
AEB cannot be used.
Even if an external Speedlite is used, it will not fire.
Continuous still photo shooting is possible during movie shooting.
However, the captured images will not be displayed on the monitor.
Depending on the still photo's image-recording quality, number of shots
during continuous shooting, card performance, etc., the movie shooting
may stop automatically.
If you plan to shoot still photos continuously during movie shooting, it is
recommended that you use a card with a higher performance than those
specified by "Required Card Performance" on page 17. Setting a smaller
image size for still photos and shooting fewer continuous still photos are
also recommended.
You can shoot still photos in all drive modes.
The self-timer can be used before you start shooting a movie. If used
during movie shooting, the self-timer will switch to single shooting.
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