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# Thread: Looooong Exposures on fp4/hp5

1. The reason I asked that question is that the best fit to the Bond data for those two films plots as two parallel lines on log-log paper. The lines will never cross.

2. I have to admit to a rash statement. Oh, well. There's a first time for everything. If I use my own equation for reciprocity correction:
tc = tm^1.62*a +tm
where tm is the value of exposure time your light meter would read and tc is the exposure as corrected for reciprocity failure, and solve for tm1 where tc1 = tc2 when film 1 has ISO 400, a=.1 and film 2 has ISO 100, a = .01, I find that when tm1 = 25793.42 seconds and tm2 = 103173.68 seconds, tc1=tc2= 1427459 seconds. That is 396.52 hours, or 16.52 days. If you had a meter that would measure such a low light values, it would only be used for photographing coal mines at midnight, or scenes above the Arctic Circle at night time.

Films 1 and 2 are closely represented in the real world by HP5+ and some of the modern films. The a coefficient I found for TMX was about .05, and TMY was about the same. That means that TMY is definitely faster than HP5+ at any metered exposure > 1 second, but so also is Delta 400.

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