Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 214)

2.
Set [Multiple exposure].
Select an option, then press < >.
To exit shooting multiple exposures, select [ Disable ].
On:Func/Ctrl
Convenient when shooting multiple exposures gradually as you check
the results. During continuous shooting, the continuous shooting speed
will decrease greatly.
On:ContShtng
Used for continuous multiple exposures of moving subjects.
Continuous shooting is possible, but the following operations are
disabled during shooting: menu screen viewing, image review after
capture, image playback, and undo last image (
).
Note that the single exposures used for merging are discarded, and
only the multiple-exposure image is saved.
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