Double exposure is all about putting bright things on top of dark areas. That’s the way it works with exposure: you can’t add a dark thing on a light background because once the silver halides are exposed you can’t add darkness (lack of exposure) to them.

If you are double exposing something against a dark background and placing it in part of the frame where there is a dark area in the other image, then you might not need to compensate in the exposure. (This is the case with a matte shot.)

But if you are exposing two scenes with a variety of dark and bright areas, it is a good idea to close the lens an extra 2/3rds of a stop.

Why close down 2/3rds of a stop?

The answer is rather subjective and it’s a little hard not to have it sound a little confusing:

You do need to close down if two bright things are being double exposed, to prevent overexposure. However, the areas of overlap should still be a little brighter than the normal exposure (otherwise the dark areas of the frame will be dark and only the bright overlapping areas will seem normally exposed). Closing 1 stop will mean that only where there is an overlap of bright areas will the image seem normally exposed, and anything overlapping a dark area of the frame will just seem dark.

2/3rds of a stop is a compromise that darkens both images (to avoid overexposure), but not so much that the bright areas of overlap won’t seem a little brighter.

Keep in mind that there is a certain subjectivity in this: You might want to emphasize one of the images through exposure. You might want to additionally compensate for differences in lightness and darkness between things you’re double exposing (to keep the brighter thing from crowding out the darker thing).

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