Canon EOS 6D Instruction Manual page 159

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 [Additive] is 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 will look
less noisy.
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].
3
Set [Multi-expos ctrl].
Select the desired multiple-exposure
control method, then press <0>.
4
Set the [No. of exposures].
Press the <V> key to select the
number of exposures, then press
<0>.
You can set it from 2 to 9 exposures.
P Multiple ExposuresN
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