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If you shoot with a camera that doesn’t have a coupled film advance and shutter cocking lever [Argus C3], the chances are you have accidentally shot some double exposures by forgetting to wind the film on after each shot. This creates overlaps and results in interesting effects that can be as equally unexpected as they are serendipitous.
Even if shooting multiple exposures deliberately results can be fairly unpredictable, and cumulative exposure in overlapping areas will result in over exposure so it is important to compensate for this. If you aim to create one correctly exposed image from two exposures in the same lighting conditions, you need to halve the amount of light hitting the film. Seeing as each stop on the camera represents double or half the amount of light that enters the camera you need to reduce either the shutter speed or aperture by 1 stop.
Correct exposure at 1/125 would need to be set to 1/250 for a double exposure (1 stop underexposed), and 1/500 for a quadruple exposure (2 stops underexposed).
Correct exposure at f8 would need to be changed to f11 for a double exposure (1 stop underexposed), and f16 for a quadruple exposure (2 stops underexposed).
This is a basic guide assuming that you are shooting each exposure in the same conditions and want equal emphasis of each exposure in the resulting image. Each exposure does not have to be equal however. In the example above, a quadruple exposure is made from 4 shots each 2 stops underexposed. 4 multiplied by 2 equals 8 stops in total. You could also shoot one 4 stops under, one 2 stops under, and two 1 stop under, as long as the aggregate is 8 stops underexposed in total.
TIP: With a coupled film advance and shutter cocking lever you can prevent the film from advancing to the next frame by holding in the film rewind button in while you wind the film advance lever to cock the shutter.