Canon EOS-1DX Instruction Manual page 190

P Multiple Exposures
Merging Multiple Exposures with an Image Recorded in the Card
You can select an image recorded in the card as the first single
exposure. The original of the selected image will remain intact.
You can only select 1 images. You cannot select 41/61 or
JPEG images.
Images shot with highlight tone priority set to [Enable], or with cropping
information appended (p.339) cannot be selected as the first single exposure.
Auto Lighting Optimizer, peripheral illumination correction and chromatic
aberration correction will be disabled, regardless of the settings of the
1 image selected as the first single exposure.
The ISO speed, Picture Style, high ISO speed noise reduction, and color space,
etc., set for the first 1 image will also be set for the subsequent images.
If the first 1 image's Picture Style is [Auto], the [Standard] Picture
Style will be set for the subsequent images.
You cannot select an image taken with another camera.
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1 Select [Select image for multi. expo.].
Select [Select image for multi.
expo.], then press <0>.
The images in the card will be displayed.
2
Select an image.
Turn the <5> dial to select the
image to be used as the first single
exposure, then press <0>.
Turn the <5> dial to select [OK].
The file number of the selected image
will be displayed at the bottom of the
screen.
3
Take the picture.
When you select the first image, the
number of remaining exposures as
set with [No. of exposures] will
decrease by 1. For example, if [No. of
exposures] is 3, you can shoot two
exposures.