How Multiple Exposure works
How much to underexpose each shot when stacking exposures on one frame.
Where to find it
Tools tab Multiple Exposure
Summary
Calculator for multiple-exposure photography. Tells you how many stops to underexpose each individual shot so the cumulative stack lands at correct exposure.
Detail
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How it works
Some cameras let you expose the same frame more than once before advancing the film. If you simply meter normally for each shot, the frame will be wildly overexposed because the light has been added together. This modal tells you how much to underexpose each shot so the final stack looks correctly exposed.
Number of exposures
How many times the same frame will be exposed. Two for a classic ghost double-exposure. Three to eight for layered effects.
The compensation
Two exposures need each shot to be one stop under. Four exposures need each shot to be two stops under. The rule is to underexpose each shot by log base 2 of the number of exposures. The modal does the math for you.
When to break the rule
If one exposure is the dominant subject and the others are dim accents (a face plus dim spark or stars), you can shoot the dominant frame at metered exposure and the accents at metered exposure too, because the dim accents add little light. The rule above assumes all exposures are equally bright.
Practical tip
Mark the back of the camera with a piece of tape so you do not advance the film by accident between exposures. Most cameras with a multi-exposure switch handle this automatically; older bodies need a careful hand.