Lost in sensitometry!

Adam Kimball (afk@uclink2.berkeley.edu)
Tue, 24 Jan 1995 23:57:06 -0800

Hello all, once again I'm full of questions, and wondered if anyone out here
could help me out. I have recently begun the somewhat intense process
of drawing film and paper curves to help me understand what kind of
negative I am really looking for. I haven't yet begun the film testing side
of the equation because I'm still unsure of what I AM looking for. First let
me relate some of my data. I tested three different developers with different
amounts of Sodium Dichromate for contrast control. I arrived at paper curves
for each type of developer with a specified amount of 5% S.D. added. I now
have about 6 curves for each type of developer. Now, assuming my test
data is correct, I would like to produce negatives which relate to my "ideal"
paper. I am "keying" on the paper curve described below. BTW, I forgot to
mention that I am doing kallitypes not platinums.

Using a Sodium Acetate developer (100g S.A. in 1000ml) with 4ml of Sodium
Dichromate added (for contrast control), I arrived at a paper curve with an
ER of 1.05 and a DR of 1.11. To calculate the IDmax of the paper curve I took
the maximum density (1.38) subtracted the IDmin (.04 plus B+F) and multiplied
this by .90.

IDmax= (1.38-.07)*.9 = 1.18

DR = IDmax(1.18)-IDmin(.07) = 1.11

I made the IDmax 90% of the maximum shoulder denisty because this was the
value that was suggested in _Beyond the Zone System_. I don't know if this
applies to the Kallitype process or not though. Does it? Anyway, after
drawing the vertical lines the IDmin line corresponded with an Emin of 2.10,
while the IDmax line corresponded with an Emax of 1.05.

SI = 2.10-1.05 = 1.05

Everything sound? Now, for the questions. How do I relate my paper SI to my
film test data? What is the relationship between average gradient and the paper
SI? Is there one? Once I have curves for my film, on what basis do I decide
which one would be used for "normal" subjects.

This question occured to me while I was thinking about
finding speed points on the film-curves. Phil Davis, in the book referred to
above, uses an "average G (with bar above)" of .5 to find a "standard IDmin".
I suspect that I DON'T want an "average G" because my paper isn't acting like
an "average" silver- bromide paper. It seems as though there is some type of
catch-22 going on because I can't find my speed points without knowing my
average gradient, and I can't find my average gradient without my speed point!
Should I just approximate the average gradient from my IDmin and IDmax values
and then tweek the numbers around if my negs come out badly? Is there a better
way of finding an average gradient than the slope of the line connecting the
IDmin and IDmax values? Or maybe a better way of finding my speed points?
This stuff gets nasty!

As you can see, I am at a complete loss. I know this is a lot of info
to throw at you unsuspecting fellows, but I'm dumbfounded. Any help would be
GREATLY appreciated, or even the recomendation of a book or two that deals
with this stuff.

Thanks a million,
Adam