Canon EOC-1DX Instruction Manual page 181

Additive
The exposure of each single exposure is added cumulatively. Based on
the [No. of exposures], set a negative exposure compensation. Refer
to the basic guide below to set a negative exposure compensation.
Exposure Compensation Setting Guide for Multiple Exposures
Two exposures: -1 stop, three exposures: -1.5 stop, four exposures: -2 stops
If [On:Func/Ctrl] and [Additive] are both set, the image displayed
during shooting may look noisy. However, when you finish shooting the
set number of exposures, noise reduction will be applied and the final
multiple-exposure image that is recorded will have less noise.
Average
Based on the [No. of exposures], negative exposure compensation
is set automatically as you shoot multiple exposures. If you shoot
multiple exposures of the same scene, the exposure of the subject's
background will be automatically controlled to obtain a standard
exposure. If you want to change the exposure of each single
exposure, select [Additive].
Bright/Dark
The brightness (or darkness) of the base image and the images to
be added are compared at the same position, and then the bright (or
dark) part will be left in the picture. Depending on the overlapping
colors, the colors may be mixed depending on the brightness (or
darkness) ratio of the compared images.
Set [Multi-expos ctrl].
3
Select the desired multiple-exposure
control method, then press <0>.
Set the [No. of exposures].
4
Turn the <5> dial to select the
number of exposures, then press
<0>.
You can set it from 2 to 9 exposures.
P Multiple Exposures
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