Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 215)

3.
Set [Multi-expos ctrl].
Select a merging option, then press < >.
Additive
The exposure of each single image captured is added cumulatively.
Based on [
No. of exposures ], set negative exposure compensation.
Follow these basic guidelines to set the exposure compensation
amount.
Exposure compensation setting guidelines, by number of
exposures
Two exposures: –1 stop, three exposures: –1.5 stops, four exposures:
–2 stops
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 the standard
exposure.
Bright/Dark
Brightness (or darkness) of the base image and added images is
compared at the same position, and bright (or dark) portions are
retained. Some overlapping colors may be mixed, depending on the
relative brightness (or darkness) of the images.
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