Here are the steps to calculate exposure for multiple overlapping exposures using different ISO ratings:
1. Determine the base ISO for one of the exposures. This will be the ISO setting that determines the overall brightness and exposure of the composite image. Choose an ISO that suits the overall brightness and contrast you want for the final image.
2. Calculate the exposure time for the base ISO exposure using the scene luminance and your camera's meter reading or histogram. This will be the longest exposure time.
3. Adjust the exposure time up or down for each additional overlapping exposure based on the ratio of its ISO to the base ISO.
For example, if the base ISO is 100 and you have two additional exposures at ISO 200 and ISO 400:
Base ISO: 100 -> Exposure time: 1 sec (for example)
ISO 200: Expose for 0.5 sec (200/100 = 2, so 1/2 the time)
ISO 400: Expose for 0.25 sec (400/100 = 4, so 1/4 the time)
4. Ensure the total exposure time across all overlapped frames does not exceed the reciprocity limit of your film or sensor. You may need to adjust ISO values or exposure times to keep within this limit.
5. When composing the final image, adjust the brightness of each exposure layer separately to get the right balance before permanently combining the images.
6. Make any final touch-ups to the brightness, contrast or color balance of the combined image as needed.
Let me know if you have any other questions!