Inverse square law and multiple flashes

[feppe]

Due to misaligned stars I have studio shoot with a film camera tomorrow, but no flashmeter or polaroids. The digital camera I could use for chimping goes only to ISO 200 @ f/22, but the film is ISO 100 and I need f/22. In case the studio doesn't have ND filters or a light meter, I need to figure out how to chimp with what I have.

So, do I have the math right: chimped shot at ISO 200 correctly exposed, flash #1 at 1 meter from subject, flash #2 at 2 meters. To convert to ISO 100 while keeping aperture constant, I'd need to increase flash #1 by 1 stop, and flash #2 by 2 stops due to different distances. Correct?

Any other ideas are also welcome. Yes, even if the math works out, I will be bracketing like there's no tomorrow.

[PeterAit]

No - you would increase both flashes by 1 stop. The inverse square law tells you how light changes as distance changes. You are not changing distances, just output.

If, on the other hand, you moved a flash from 4 feet away from the subject to 8 feet away, you would then need to increase the output by a factor of 4, or 2 stops.

[Sheldon N]

So, do I have the math right: chimped shot at ISO 200 correctly exposed, flash #1 at 1 meter from subject, flash #2 at 2 meters. To convert to ISO 100 while keeping aperture constant, I'd need to increase flash #1 by 1 stop, and flash #2 by 2 stops due to different distances. Correct?

I don't think so. You've already accounted for the difference in distance when you set flash power to begin with. Just add 1 stop to each light.

[Ken Bennett]

Peter is right, both flashes are increased by 1 stop (double the output) to account for the decrease to ISO 100 film.

You are not changing anything else about your setup other than the sensitivity of the film.

[feppe]

Thanks, that makes sense!

[feppe]

Update: they had a 4-stop ND filter at the studio, and I got some some usable shots. I'm going to buy one for myself so I'll have a backup next time the stars are misaligned.

Thanks again for the tips!

[mediumcool]

No - you would increase both flashes by 1 stop. The inverse square law tells you how light changes as distance changes. You are not changing distances, just output.

If, on the other hand, you moved a flash from 4 feet away from the subject to 8 feet away, you would then need to increase the output by a factor of 4, or 2 stops.

Inverse square rule is accurate only for point sources.