RE: Digital Negs - RGB vs CMYK

From: Keith Gerling ^lt;[email protected]>
Date: 12/03/04-01:21:22 AM Z
Message-id: <>

I use variations of RGB and CMYK to fit my needs. I guess you could say I'm
one of those geeks that is happy to have the dot gain and ink limit
controls. Early in my gum-printing education, I was fortunate (I didn't
think so at the time) to do some custom printing for a Photoshop-savvy (and
gum-ignorant) commercial photographer. He would examine my results and say
things like "do the EXACT same thing, but decrease the dot-gain by 5%". It
made big differences, and they weren't "arbitrary". Since those days, I've
slackened up on having to have precise separations negs, preferring to
direct my tweaking to the gum printing stage. Still, when dealing with a
problem picture that just doesn't turn out right, it's nice to be able to go
into image/calculations, do a (for instance) linear light blend between the
red and blue channels at 50%, "clamp" it with curves, make another neg, and
pound some more magenta into a picture to really make it snap. These
changes can be subtle, or they can be hideous. But it's nice to have these
options at one's disposal. Some people will only drive a car with an
automatic transmission. I've always preferred manual.

My advice to those just starting: if you want to make things easier for
yourself (and become a better gum printer in the long run), make perfect
photographs and always use RGB separation negs. You'll be far happier, But
if you are like me, and your photographs are'nt "perfect", it might be worth
you while to familiarize yourseld with some basic channel manupulation.


-----Original Message-----
From: Katharine Thayer []
Sent: Thursday, December 02, 2004 2:11 AM
Subject: Re: Digital Negs - RGB vs CMYK

Jim Morris wrote:
> Hi,
> For the purposes being discussed, I believe using either RGB or CMYK
> files can provide similar, if not the same, results. The only real
> difference between the two, for the purpose of separation films, is
> that RGB files have three channels and CMYK files have four channels.
> In my opinion, using RGB negs. is a simpler approach, but CMYK offers
> more options if you are willing to explore the settings. It's true
> that if you simply use the "default" CMYK settings in Photoshop you
> will need to use the K printer film as it will carry a significant
> amount of the image's information, but you can "choose" to generate a
> CMYK file that only carries information in the CMY channels and leaves
> the K channel empty, in this case it may be simpler to use RGB.

Hi Jim,
Yes, you can turn the "black generation" setting to "none" but that
doesn't make the separations the same as you would get from inverting
the RGB file, because the other default settings (ink colors and ink
limit especially) alter the curves from straight CMY, especially for
the magenta and yellow separations.

I agree with your main point that CMYK gives you more settings to play
with, but in the end all those settings do is alter the curves in (to my
un-CMYK-savvy eyes) rather arbitrary ways that make sense for the
commercial printing presses, inks and papers they were intended for but
don't translate in a straightforward way to our process IMO. It seems to
me more straightforward, if you want to change the curves, to work
directly with the curves rather than changing settings with names like
"dot gain" "UCA" "Ink limits" and trying to judge how the different
settings will change the curves for gum printing.

But that's just me, and as I've said many times before, I think it's
whatever you're used to and whatever makes sense to you that's right,
and someone who is used to working with the CMYK settings and can get
the results they want from it-- more power to them. There's different
ways of approaching gum printing and whatever works for you is what
works, has been my mantra from the beginning of time. I would never
fault anyone for using CMYK, that's never been my point when I say that
CMYK doesn't make sense to me. I'm just saying it doesn't make sense to
ME, I'm not saying it shouldn't make sense to anyone. I think what Keith
does with his very sophisticated manipulation of CMYK, for one example,
is amazing and wonderful, and I sure wouldn't want anyone to think that
I think otherwise.

All I've been about when I get into one of these CMYK threads is to say
why *I* prefer CMY separations generated from the RGB file, and the web
page is simply a way of putting all that stuff in one place so I can
just point to it next time rather than saying the same thing over and
over and over again to people who are determined to tell me that I don't
understand color science or that it is impossible to get realistic color
balance from RGB separations, or whatever. As I said on the last go
round of this topic, there's nothing that makes people angrier than me
saying that separations by CMYK for gum printing don't make sense to me.
Well, I'm sorry, but they don't. But let me say one more time that when
I explain why I do what I do, I am NOT saying that I think everyone
ought to do it the way I do. Okay? Okay.
Katharine Thayer
Received on Thu Dec 2 23:20:17 2004

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