ExpoDev is very unique in that is uses highly accurate and test-produced reciprocity data for over 90 film and developer combinations. Phil Davis did the work of testing each and every film developer combination using selected exposure times up to 1,000 seconds or roughly 16 minutes. He also field-tested these results to ensure their accuracy. This is far above and much more specific than the data that film manufacturers publish for their films and developers, not to mention that ExpoDev's reciprocity data covers film and developer combinations across different manufacturers. Correction is not a one-size-fits-all curve; it is very unique to each film and developer combination.

What is meant by "selected exposure times" though. What this means is, when you tell ExpoDev that the shutter time you want to use is anywhere within the range of 1 to 1,000 seconds,  ExpoDev would then apply reciprocity failure correct and give you a corrected exposure time to use. This corrected exposure time may be a small adjustment for a selected exposure time of a few seconds or it may be several hours for a selected exposure time of 16 minutes (from 1 to 8 hours, depending on which film and developer combination you use).

There is a second part to reciprocity failure correction though and that is that the overall resulting negative density also changes with extended exposure times. Unlike exposure times negative density can change either direction, either increasing or decreasing depending on the film and developer combination used. ExpoDev's reciprocity data for these film and developer combinations includes how development time should be adjusted as well. This reciprocity data is used to apply an adjustment to the Average Gradient that is calculated as part of the exposure. It is this Average G value that is used to look up the development time that should be used to produce a negative that matches the target density desired (so that printing or scanning remains consistent and easy across all of your negatives).

Each Film Profile that you import into ExpoDev contains a two-part code expressed as a letter/number combination (e.g. D4, A6, etc…), The first part is for the exposure time adjustment code and the second part is the development adjustment code. For the exposure time adjustments there are 5 codes, A-E; for development adjustments, there are 7 codes, 1-7. When you export film test data from Plotter for Windows you select which code to use for that film/developer combination (Plotter has a table for all the film and developer combinations that were tested). Since you get to choose the code to use when you export your film test profile this also means that you can override the codes if your working processes need adjustments. You can even choose to export without any codes at all which will cause ExpoDev to skip reciprocity adjustment altogether.

In the next article I'll cover ExpoDev's adjustable Depth of Field calculator.

Until then I'm happy to say that ExpoDev version 1.0 has reached the final testing stages and if all goes well, it should be available in the Apple iTunes App Store very soon.

Comments (1) -

2/29/2012 12:31:29 AM #

Can't wait it! I'll be one of the first users Smile

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Flux and Mutability

The mutable notebook of David Jade