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Old 04-17-2019   #10
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Join Date: Jan 2014
Location: New York City area
Posts: 1,210
I've got thousands of bulbs of many types. You set to M, usually 1/30th sec, test at 10ft with specific film ISO, and with a specific flashgun and reflector to compute your guide number. Then when you shoot at a specific distance you can compute your f/stop. f/stop = Gn/distance
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