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Adjusting Negative G-Bar
by Phil Davis

The WinPlotter now includes all of the film/developer files that were described in the April 2002 issue of the D-Max newsletter. I produced these files using an ES value of 1.05 (equivalent to an average grade #2 paper) and ran the tests at 70° F. Presumably these files should be usable by any Plotter user who matches these ES and temperature values, and follows the usual processing procedure with the BTZS film tubes.

But, for reasons that aren’t obvious, some people have reported that these test data result in overdevelopment. If development errors of this sort are a consistent problem, the test data should still be usable and reliable after they’re been suitably adjusted.

The most likely cause of over- (or under-) development is temperature difference, probably due to a difference in thermometer calibration. Whether this is the case or not, adjusting the processing temperature is a useful way to make the necessary development adjustment. As a general rule of thumb, at least within the 65°–75° Fahrenheit range, you can use this little equation to estimate the temperature change that will provide a desired change in G-bar.

Subtract the G-bar value you want (G1) from the value you have (G2), add 1, and divide the temperature by this sum:

        New Temp (F.) = Old Temp / ((G2 – G1)+1)

 For example, if you process at 70°, aiming for a G-bar of 0.5 (G1), and you estimate that the resulting negatives are averaging about 0.55 (G2),

 New Temp = 70 / ((.55 - .5)+1))  =  0.67°F.

Another practical adjustment method that may be even easier for you, is to change the ES value you enter into the Expo/Dev program. In the middle of the ES and G-bar ranges (avoiding extreme values, in other words) G-bar changes at a rate of about 0.05 for each change of 0.1 in the ES value. For example, to reduce G-bar from 0.55 to 0.45 (change of 0.1), substitute ES 0.9 for an existing ES value of 1.1 (a reduction of 0.2).

Remember these adjustment methods can provide useful approximations that will point you in the right direction, but don’t expect great precision. Once you’ve discovered a satisfactory temperature adjustment for any one of those “Library of Film Curves” files, it should apply to all of them.


© 2002 Phil Davis