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06-14-2015, 06:06 PM   #1
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Join Date: Jan 2015
Location: Ottawa, ON
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Looking for some help with flash + ambient math

I want to experiment a bit with some Agfa Rollei IR film I have in my camera at the moment, mixing flash in with ambient for a silly outdoor portrait idea. The film is ISO 400, but I'm using a R72 filter, which in theory is a 5 stop reduction in exposure. I'm planning on doing the photo on a bright summer day, and underexposing the background by 2 stops, but filling in my subject with a flash. So I the overall scene exposure (exposed 2 stops under) can be done with F8, 1/60. But I'm not sure about flash distance to get my subject properly exposed.

If I had a GN 115 flash, what distance do I need to roughly have it from my subject to gain those last two stops... I think it's about 14 feet, but could be mistaken.

06-14-2015, 07:27 PM   #2
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I get about 16.5 feet, though I would "bracket" your flash-to-subject distance if the shot is important. Try 13, 16, and 19 feet. You might also want to "bracket" your flash/ambient balance, in case you decide you want a more flashy or more natural look, or possibly, in case the flash's IR contribution is different than natural daylight. To do that, try 1/45th @ f/9.5, and move the light to 13 feet, or 1/30th @ f/11 and move the light to 10 feet. Good luck!

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