RE: CMYK separations on Adobe Photoshop

From: Keith Gerling ^lt;>
Date: 11/05/04-01:11:26 AM Z
Message-id: <>


Recent events have turned my brain to mush, and it's possible I've missed
the point entirely. This is what I believe: 1) I didn't read the entire
thread regarding "CMYK Separations in Adobe Photoshop", but saw one, read
it, and found myself responding, 2) I've used both CMYK and RGB separations
to make gum prints and have been successful with both, 3) the preference of
one over the other is most likely a matter of taste and not the superiority
of one over the other, 4) regardless of whether one uses CMYK or RGB as the
color separation tool in Photoshop, one will end up with (in Photoshop
terms) "Channels" that contain information that one can use to approximate,
via pigment, the color information that was present in the original digital
file, and 5) the information contained in these channels is black and white.

I don't understand the concern over orthocromatic lith film, because in no
case will anything other that white light be projected onto it. I ask you
to explain in more detail why "red response" in the copy negatives would be
a factor. I'm not well acquainted with using inkjet negs, but again, I fail
to see how the spectral inks in an Epson 2200 could play a factor once the
color component has been neutralized via the separation process.

Although the information contained in RGB is more "accurate" than that
contained in CMYK, in either case only approximate renditions are possible
using pigment due to the inability of pigments to convey the full gamut of
the optical spectrum. It is my opinion that the deference's between CMYK
and RGB separations will be well within the (rather large) standard of error
that is inherent in gum printing.

While I have an interest in science, I must confess that my temperament and
work ethic does not dispose me to be a champion of pure scientific method.
I was hoping that I could make a limited contribution by offering up for
examination two gum prints with nearly identical source material, one
CMYK-derived and the other with RGB. I lack the necessary tools to perform
this task according to scientific standards. For one thing, my process
renders it impossible to expose the CMYK and RGB components simultaneously
for the mere reason that this would require the negatives to be perfectly
equal in density. There's no way I could accomplish that using two pieces
of lith film developed separately.

I withdraw my offer. It was naive of me to make it in the first place.


PS. But I think I may go ahead with my original plan. It might not be
science, but it will be challenging and fun. Besides, now I'm curious..

-----Original Message-----
From: Joe Smigiel []
Sent: Thursday, November 04, 2004 6:36 PM
Subject: RE: CMYK separations on Adobe Photoshop


If I am understanding you correctly I think there is a glitch in the
test you propose. If by "lith film" you refer to an orthochromatic
emulsion, then the spectral sensitivity of that film will distort the
red response in the copy negative(s). I think you'd have to use a
panchromatic film to retain something close to the original color

Of course, for all I know the CMYK inks that the Epson 2200 uses when I
make digital negative separations might also be causing a spectral shift
even though the negatives look neutral in color.

In any event, to make the comparison more attuned to the scientific
method, you'll want to hold as many variables constant as possible. As
a result, one thing to do is to divide the printing substrate in half
after it is prepared and coated evenly with emulsion. Then,
simultaneously print both halves using the same exposure and process
them together. You can't always control everything precisely, but you
can get close and at the same time make it similar to your normal method
so that the results have some practical rather than purely theoretical


>>> 11/04/04 9:52 PM >>>
Soon after making this reckless statement, I realized how much work I
gotten myself into. (My negatives are on lith film made in the
old-fashioned way: in a soon-to-be cold darkroom, and I hate it.
work, that is). So this is what I'll do: I will make a diptych where
half is of the same subject shot at the same time. In other words, I am
reneging on the offer to make two versions with identical source, but
they'll be pretty darn close to being identical. I'm thinking of
on the order of a nude with two similar poses side by side. And, I
if this effort is going to result providing any valid new information, I
ought to stick with a conventional color palette and play by the rules,
i.e., attempt for a "true" representation of the original shot. And
I'll record and publish the steps in the process.

Anybody have any other suggestions? I'm new to this whole "scientific
testing" stuff.

-----Original Message-----
From: []
Sent: Thursday, November 04, 2004 4:26 PM
Subject: Re: CMYK separations on Adobe Photoshop

Keith wrote:
I will personally
> make two gum prints, one using CMYK negatives and the other using RGB
> negatives. OK? I am confident that this demonstration will prove
> neither has an advantage.

Can we also get the details of your conversion to CYMK on Photshop (UCR,
Received on Thu Nov 4 23:10:42 2004

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