I've been using Ilford's reciprocity curve for Delta 100 with great success. Appears to be spot on in my experience.
Thomas
This is actually in close agreement with Kodak's suggestion for Tri-X films (Kodak Data sheet F-4017, page 2) which says essentially:
metered - adjusted
1 sec - 2x (add one stop)
10 sec - 4x (add two stops)
100 sec - 8x (add three stops)
I think people have trouble extrapolating this to longer times and feel uncomfortable with the coarseness of the table.
It is easy to express this relationship in an simple equation...
ta = tm * 2(1 + log10(tm))
which can be re-written:
ta = 2 * tm (k + 1), where k = log10(2)
or, replacing k+1:
ta = 2 * tm (1.3)
This is probably a pretty good starting place for most films.
Last edited by BradS; 21-Dec-2013 at 12:15. Reason: typos
Formulas are nothing but a guesstamate. You really should do your own testing. I have found Ilford's data to be a bit over the top. I have been using my own tested data in the field for 16 years for HP5 and FP4 and have been satisfied. I also have data for Delta 100 somewhere. If I dig it up, I can post it if you like.
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I concur. I hope people understand that all that I have done is give an equation for the relationship provided by the manufacturer. In Ilford's case, by choosing three points from the graph in their datasheet and writing the equation for the parabola that passes through those three points and in the and similarily with the Kodak data, all I have done is express in an equation what they say in word and in the table. There is no magic to this. The equations are no better than the data from the respective manufacturer.
As with so many things in photography, take this stuff as a starting point and develop (pun?) numbers, tables, graphs, whatever that works for you...based upon your own empirical evidence.
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