Reciprocity means that certain films need more light than expected if you use long exposure times. (Schwarzschild Effect)
The values differ from film to film and you need to use tables, an app on your phone or a calculator as linked below.
With Foma 100 as an example you see that this film is not very great for long exposures (pinhole)
Some expected Foma 100 times, rounded, using the formula above:
time – time with reciprocity failure
5 seconds – 26 seconds
8 seconds – 56 seconds
10 seconds – 80 seconds
100 seconds – 1603 seconds (27 minutes)
600 seconds (10 minutes) – 3 hours
30 minutes – 9 hours
1 hour – 18 hours
Ilford films behave better:
https://www.ilfordphoto.com/wp/wp-content/uploads/2017/06/Reciprocity-Failure-Compensation.pdf
Here you see the difference pretty good:
Many of us just guess with the kodak table:
https://mkaz.com/film-reciprocity-tables/
http://www.jla-analog.net/reciprocity-failure-calculator/
https://retro-pixel.com/film-reciprocity-tables/
By the way,
in an interview with Andrew Bartram in the large format podcast Clyde Butcher explained something very interesting:
He uses reciprocity deliberatly to increase contrast in his shots.
That concept was new to me and very interesting, so maybe the Fomapan film can be of execellent use for increasing contrast…
Dear Gerhard,
I read about the increase of contrast due to the reciprocity failure in Bruce Barnbaum’s Book The Art of Photography. He provides nice explanation of this effect.
It is also mentioned in table of pg. 75 of Using the View Camera (by Steve Symmsons). If you use the zone system (N+1, etc ) in the development, you need also to adjust the development time (see p. 157). The interesting stuff is that if you increase dev. time (e.g., in N+1, N+2, etc.) you increase gain size. The Recip.Fail.Effect, on the contrary, does not increase grain size.
Cheers
Luis
Thank you Luis,
this I read about but never tested it. Did you?
Important Information indeed.
Cheers,
Gerhard