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Most of Phil Davis's film tests are done with the times 4 min. - 5.6 min - 8 min - 11 min - 16 min. This is, of course, a geometric progression with a square root of two multiplier. One only has to look at the test results to see that this progression generates a highly useful spectrum of curves. But aren't the relevant chemical processes additive rather than geometric in nature? Unlike exposure, for example. (It has been pointed out by Mr. Davis and, I believe, Dickerson and Zawadski, that for just this reason an arithmetic progression is a rather silly way of making a test strip.) But, if the process we are monitoring is additive in nature, why use a geometric progression of times?
So, for example, if I wanted to test five films at times between 4 and 22 minutes, would it be reasonable to use the progression 4, 8.5, 13, 17.5, 22? (start at 4 minutes and add 4.5 minutes for each subsequent time up to 22 minutes.)
As you might suspect, I did not pick this example at random. I would like to try my usual film, HP5+, in a new developer, DD-X, but at a slightly higher dilution than Mr. Davis used in his test, so that I can generate Gbar data down to about .3. However, in Mr. Davis's test at 1:6 the 16 minute curve appears to be barely suficient to straddle the ISO triangle. To generate the nececessary data at the higher dilution I will use (1:7 or 1:8), I thought it would be prudent to continue on to 22 minutes. Being naturally lazy, however, it then occurred to me that if I could adjust the times I might still only develop 5 films, rather than 6. Hence this question.
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