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:
(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)) =
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.