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

Katharine,

say we'll take a little ride and explore a few things together. I think we
agreed a while back that a gum emultion react to light in some proportional
manner, more light equal more pigment get caught in the matrix of gum that
is created by the interaction of dichromate and gum at the molecular level.

Below you say: "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". Ok, your right it is absolutely true that I or
anyone else for that matter, can't obtain a continuous tone "gradient" from
Dmax to Dmin using a curve in a digital world, because in this world there
is only a limited number of steps (in the general sence) one can achieve. If
we speak of an 8 bit black and white image the maximum number of step is 256
(0..255). In the digital world you can increase this to about 16 million
step if you use a 24 bits color image.

Basically, you can stop me about here if you think that 16 million steps if
not enough to be called continuous in practice but in theory I agree with
you it is far from continuous.

Now another point, say we agree that as you said below only 10-12 step out
of 21 can be obtain from the best emultion we can make in a single exposure.
In terms of negative density this mean a maximum of 12 steps * 0.15/step =
1.8, wow! This is far out, it would seem only carbon and silver gelatin can
better this according to Sandy. But lets say to please every one that we use
is figure or 1.20 above paper white. I don't know about you but I don't own
a densitometer, instead I use my Gossen light meter equiped with a fiber
optic probe. The best I can read with it is about 1/6 of a stop or about
0.05 in density which is obviously insufficient but lets say we consider a
density resolution of 0.001 instead, this is about 3/1000 of a stop. Can we
agree that this is plenty enough to be call continuous in practice, you say
NO because this is only 1200 steps from 0 density above paper white to 1.200
above it and I kind of agree with you, we can do much better, say we use a
0.0001 resolution, this would make 12000 steps which is much better don't
you think.

I'll admit here, I haven't read the PDN book nor Don's book, both of which
are about making digital negative and lets say we wont limit ourselve in
anyway and try to reach for the top, meaning we'll have to find our own way.
As I said earlier, a color image needs at least 24 bits of information to
represent a sufficiently large amount of all the colors we can observe in
nature or even man made ones. This is 16777216 different colors, this is way
to much, we only need 12000 out of these but is it possible to find 12000
colors out of these 16 million such that each of them give us a constant
0.0001 difference between each consecutive color when ordered properly of
course. From here, there is a short way and a long way, the short way is
accepting for a fact that it is possible to generate at least 12000
increasing density steps with a constant step size of near 0.0001 value in
density. The long way is to proove this using the Beer-Lambert law and this
would take more time then both of us have for now.

Another mistake I've made is I use "the curve" where I should have used "the
curves", imagine for a moment that I choose a Hue at max saturation and max
lightness that when given to the printer these values produce a density of
1.2000, keeping this same Hue we can vary both saturation and lightness in
such a way as to produce a linear decrease in density with step of 0.0001
from 1.2000 to 0.0000 and obtain our objective 12000 steps of which and
unless you have very sophisticated equipment you wouldn't be able to discern
a large amount either because of the limit of your instrument or because the
printer hardware software isn't configured to produce various densities but
various colors as best as possible mechanically. We can find all the excuse
we want but in the end I have absolutely no reason to believe it is not
possible to have a printer produce a large enough number of density values
that the 256 different densities we can obtain from an imagesetter would
seem like kid stuff.

I think you would also agree with me that controling the density
distribution on a negative is effectively controling the distribution of
exposure we give a certain print and indirectly we also control the tonal
distribution as well. If you limit yourself to 8 bit or 256 distinct values
of which only a partial number would be useful to control our density
distribution then I'm with you, there is no way I can produce any tone I
want. It's like using a boxing glove to clean my nose. But if like me, you
think that out of the 16 million plus colors a printer can produce, the
exact figure is probably much less then that but even say 10% of this is
still well above the million mark, we probably need only a relatively
limited number of distinct tone to give the illusion of continuous tone. I'm
sure the actual number is below 250 as examplified by imagesetter negatives
where some values produce a much to high density to be useful.

Still convinced I can't produce a negative with a large enough number of
density values in the appropriate range to produce a print with an apparent
continuous tonality gradient. Well think of this, with my Hue example I
purposely limited myself to a subset of the potential 16 M colors because I
used a fix Hue. Now there is a software call Gimp and with this little
fellow I can create program that can use all 24 bits or all the 16 million
colors. There would be obviously a long study to learn the relationship
between these 16 million colors and the density they each produce. Enough of
this nonsense, I'm sure we don't need no where near a million colors or
densities and I think I've made clear enough that if an imagesetter can
produce fine prints and this with less then 256 distinct densities, making a
digi-neg from a 24 bit color file and requiring only 256 distinct densities
from potentialy of much more, should be a walk in the park even in a wheele
chair.

Regards
Yves

----- Original Message -----
From: "Katharine Thayer" <kthayer@pacifier.com>
To: <alt-photo-process-l@usask.ca>
Sent: Tuesday, May 02, 2006 2:50 PM
Subject: Re: Back-exposing on plastic (was: Re: Gum transfer

>
> On Apr 28, 2006, at 8:44 AM, Yves Gauvreau wrote:
>
> > Katharine,
> >
> > my first reply on this topic was probably the cause of the
> > misunderstanding,
> > when I read it back now I see what you mean. With the last one I
> > thought I
> > made all this as clear as I can but I'll try again. If whatever you
> > do back
> > exposing your print fails to give you a satisfying tonal "delicacy"
> > as you
> > put it, may be applying a different curve would help.
> <snip>
>
> > I would certainly claim that if you maintain every variables fix
> > ie. you
> > don't change anything from print to print except the curve applied
> > to the
> > negative, you can basically obtain any tone you want between the
> > Dmax and
> > the Dmin of the print.
>
>
>
> 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).
>
> And in fact, although this is a slightly different issue, Chris's
> "correct curve" loses some of the tones she had in the uncurved
> print, like the details in the shadow side of the building, at least
> that's how it looks on my screen. I would never use a curve that
> extreme, because I don't like contrasty prints; I prefer more tonal
> sublety and smoothness. (I never liked Ansel Adams' work until I
> discovered his earlier work, which had a lot more tonal subtlety to
> it). This is not a criticism of Chris's choice of print contrast, at
> all, or Ansel Adams' later preference either, only to make the point
> that what is a "correct curve" is a very subjective thing, in
> addition to the more important point that you can't get all the
> between tones of a long tonal scale in gum just by adding a curve;
> gum can only do what gum can do.
>
> Katharine
>
Received on 05/02/06-04:11:09 PM Z