Re: Back-exposing on plastic (was: Re: Gum transfer

Katherine,

I am not sure I understand your statement below. I understand that gum is a
short scale process (requires a negative of a shorter exposure scale, like in
the range of log .9 maybe to log 1.2, give or take a log or a twig—or from
your example below, between log 1.5 and log 1.8?

I also understand that no curve on a digital negative is going to change the
exposure scale of a given mix, and certainly the curve is not going to
INCREASE the printed gum DMax.

What I am not sure I understand is if you are saying that for a given mix
of gum printed with one coat, changing the shape of the curve will NOT allow
you to get whatever tones you want between the DMax of this given mix (one
printing) and paper white? And further that you can only obtain some of the
tones ? Which ones?

Let's assume that a digital negative can have a maximum of 256 possible tones
of density. (If that is ok with you) and that a radical curve is going will
reduce the number of tones from 256, so let's consider a not so radical
curve. Let's also assume that the negative matches the exposure scale of the
gum mix before the curve is added.

Best Wishes,
Mark "Gum Impaired" Nelson
Precision Digital Negatives--The Book
PDNPrint Forum at Yahoo Groups
www.MarkINelsonPhoto.com

In a message dated 5/2/06 1:52:38 PM, kthayer@pacifier.com writes:

> Yves, I hope seeing Chris's print comparison will help you finally 
> grasp what I've been saying.  No, you cannot, once you've got the 
> right emulsion,  "basically obtain any tone you want between the DMax 
> and the Dmin of the print"  by adding a curve. With gum, you can 
> obtain some of the tones running between Dmax and Dmin,  but not all 
> of them, not by a long shot, (this is what we mean when we say gum is 
> a "short scale" emulsion, or that no matter what you do, you can't 
> get more than 10-12 steps on a 21-step for gum in one coat by the 
> usual method-- that's assuming a normal pigment load; you can get 
> more, but paler, steps with a lighter pigment load, or fewer, but 
> darker, steps with a heavier pigment load).
>
Received on 05/02/06-01:51:41 PM Z