From: Larry Roohr (lrryr@attbi.com)
Date: 03/21/02-08:13:39 AM Z
David,
You should start with a step wedge negative with no curve applied, make your
print, then make a curve that equates 0% in your image file to max density
on your print, 100% to min density, and even steps in between. I'd guess
that the .acv file you have is exactly this for someone else's process and
you need to repeat the exercise for your own. I measure the negative and
make them for specific desired density's, this assumes the print process
doesn't need any special treatment and a nice linear negative is what's
desired.
If your lucky enough to have a densitometer I've written an excel
spreadsheet that does the interpolation for you. Basically you make your
print, measure the step values, input them to the spreadsheet, it outputs
values for a new curve. Let me know if you want it and I'll send it to you.
Larry
-----Original Message-----
From: David Oliver [mailto:david.oliver@xtra.co.nz]
Sent: Thursday, March 21, 2002 1:18 AM
To: alt-photo-process-l@sask.usask.ca
Subject: Sensiometry with Digital Negatives
I recently ran some tests on my new UV light/paper/cyanotype setup with
digitally-produced step wedges. I was trying to determine the minimum time
to produce full D-max at the 95% and 100% steps. I used the Stephen
Chalmers cyanotype.acv curve to adjust my negative's contrast.
I got a discernable range of steps from 30% to 100%, which I understand
isn't too bad for cyanotypes, as they do tend to have quite limited contrast
range. My question is this - instead of applying the cyanotype.acv curve to
a "normal" negative that has had its input levels tweaked to show a full
dynamic range (roughly 0% to roughly 100%), why not start with a negative
that is tweaked to cover a range of 30% to 100%. After all, any parts of
the image that have highlight detail between 0% and 30% will be "blown out"
and show as the same brightness. So for my setup, 30% is pretty much the
maximum discernable whiteness.
Having adjusted the negative to work with the limited range of my cyanotype
setup, the cyanotype.acv curve (or similar) would then be applied to the
image, so that the steps from 30% to 100% were well spaced.
Second, I have been setting the output levels in Photoshop to K=5% and
K=95%. Could I acheive what I am suggesting above I I set K=30% (or better
35%) and K=95% for cyanotypes.
I am assuming that I can't get a greater range of contrast due to physical
constraints. But does anyone know if I should be able to create an
adjustment curve that acutally allows me to deliver a cyanotype step wedge
that prints the full range of steps form 0% to 100%. My guess is that
theoretically this might be possible, but that stretching the histogram
might cause posterisation problems.
David
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