Canon EOS-1D X Mark III Advanced User's Manual page 219

Merging Multiple Exposures with a RAW Image Recorded
on the Card
You can select a RAW image recorded on the card as the first single exposure. The
selected RAW image will remain intact.
1.
Select [Select image for multi. expo.].
2.
Select the first image.
3.
Take the picture.
Turn the < > dial to select the first image, then press <
Select [OK].
The file number of the selected image will be displayed at the bottom of
the screen.
When you select the first image, the number of remaining exposures as
set with [No. of exposures] will decrease by 1.
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