RE: Lost in sensitometry!

Bjorn Andersson Popular Historia (POPHIST@gemini.ldc.lu.se)
Wed, 25 Jan 1995 22:00:37 +0100

Dear Adam

I have also nearly killed myself over Phil Davies book! But
that was quite a few years ago. Now I think I have a more sober
view on applying sensitometry to alt-processes.

Drawing the papper curve will help a lot. It will give you a
good pointer as to what to excpect from your process. I gave up
on trying to print on the Guilleminot POP-papper after seeing
its curve, realising that I will never get good highlights on
the papper without dratic measures.

Your reasoning on the DR seems OK. I myself would calculate the
DR as follows. DR=(Dmax-Dmin x 0.90)-0.04 which would give
DR=(1.38-0.07 x 0.9) - 0.04 = 1.14

Your SI=1.05 (I saw your correction but it does not matter to the reasoning)
indicates that the "ANSI" grade of your kallitype papper is
about 2. That is a "normal" grade for photographic papper. (SI
of 1.50 gives a grade of 0).

So what you need is a "normal" kallitype negativ that would
contact print on photographic papper of grade 0. I think it is a good
idea to callibrate your negs first on photographic papper of the same
grade (draw your curve) as the process your are working with.
When you get it callibrated on photo-papper it will be very close to
the best (sensitometric) neg for your process.

Under ideal conditions your SI of the paper should be
identical to the films DR.
DR=G x ER
G(average gradient)
ER (Subjects luminostity range) An average luminosity range
would be 7 steps which is 2.1.

Kodak has its own definition of the average gradient which it calls
Contrast Index (CI).

So how do you get your negs to fit the papper curve? Well a
good starter is using the data sheets you get with your film.
Kodaks F5 "Proffesional black and white films" is a good reference for Kodak
films. The CI vs time curves will tell you a lot on how the films
behave. Sometimes you will find the curves to short for the
processes (they don't reach the CI you need)
you are working with, but it often works just to make
them a bit longer yourself aproximating the continuation.
So for your (N) development you need to divide your SI with 2.1
(which is the ER for a normal subject) to get your contrast index(CI).
A good idea is also to multiply your SI with a factor of 1.1 to
compensate for lens flare etc.

This gives you:
(1.1xSI)/2.1=CI

In your case with SI of 1.5 it would give
1.1x1.5/2.1=0.79
Looking at the filmsheet for Kodak Tri-X pan (the only sheet I had handy)
this gives me a N dev time of 14 min in D76.
All the N- and N+ times are as easily calculated.
The times arived at using this method are very good starting points.
It should only take a few trails to get it right.

As to the speedpoint I wouldn't worry to much. As most processes are
low kontrast you will be developing the film to a higher contast and
you are gaining speed. So for most normal film/developer combinations
you are getting the stipulated speed. Once again assuming good
equipment. I use Tmax 400 at 400 ASA (for all my N+ and N-) for platinum.

I don't know if I have anserwed your questions but my advice to
you is to try to keep it simple. Going to deep into
sensitometry will take a lot of time which could be used making
prints instead.

Here are some of my SI results with diffrent papper curves,
the corresponding papergrade is also given.

Salted papper SI=1.8 papergrade=-0.5
Cyanotype SI=1.1 papergrade=2.0
Kallitype SI=2.1 papergrade=-1.5
Platinotype SI=1.3 papergrade=1.0
Palladiotype SI=1.5 papergrade=0.0
Carbo SI=1.8 papergrade=-0.5
Gum SI=1.0 papergrade=2.0
Oil SI=1.3 papergrade=1.0

These values can be used as starting points but as you all know
the variables involved in these processes are so many that
there are no "true" values.

If you would like to Email me the sensitometric values of one
of your curves and also the values of your stepwedge I could give
you some more feedback on your own curves.


Bjoern Andersson
Email pophist@gemini.ldc.lu.se