Dan Margulis Applied Color Theory

Converting CMYK to CMYK

   Date: Thu, 15 Jan 2004 12:19:07 -0600
   From: Cliff White
Subject: Converting CMYK to CMYK

I just pre-pressed a bunch of images for our magazine and converted to a custom CMYK profile based on what someone at our printer suggested. I've just found out that the information was a bit erroneous and now have a new custom CMYK target. The big issue is that we had the dot gain set way too high.

So, I need to reconvert the images.  What is the quickest and easiest way of doing this?  The current CMYK images are untagged.  I have the original RAW files, but no RGB file with my color corrections and adjustments, etc.  In the interest of time, I don't want to go back and redo all of them, so can I simply reconvert the CMYK's?  If so, how do I do this?  Open up the file and use "Convert to Profile" to assign the new target?  Or do I need to convert back to RGB, before converting to the new CMYK target, and if so, will that degrade image quality or screw up the color balance somehow? I'm a little fuzzy on this, so any help is appreciated.
 
CLIFF WHITE
Staff Photographer
Missouri Department of Conservation
(573)522-4115 ext. 3854
P.O. Box 180
Jefferson City, MO  65102
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   Date: Thu, 15 Jan 2004 14:42:27 -0500
   From: Brian Pylant
Subject: RE: Converting CMYK to CMYK

I just pre-pressed a bunch of images

I never knew that was a verb...
8^)
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   Date: Thu, 15 Jan 2004 11:37:42 -0800
   From: Rick Gordon
Subject: RE: Converting CMYK to CMYK

I had been under the impression that it was safer to convert CMYK-RGB-CMYK or CMYK-Lab-CMYK, but very recently ran some comparison tests (using Photoshop CS) going from CMYK generated by a custom profile to for a Chinese printer to U.S. Web Coated (SWOP) v2, and found that the files from a direct conversion and an indirect conversion were identical. So I'm wondering whether perhaps Photoshop CS had improved in this regard, or if it was not really the problem that I had thought (and heard) that it was, or if other situations could demonstrate a more significant difference.

Rick Gordon
 
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RICK GORDON
EMERALD VALLEY GRAPHICS AND CONSULTING
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WWW:   http://www.shelterpub.com
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   Date: Thu, 15 Jan 2004 15:29:01 -0500
   From: Preston Earle
Subject: Re: Converting CMYK to CMYK

My guess is that you need to ASSIGN the old profile to the images and then CONVERT to the new profile. You could then save the files with or without the profiles attached. Simply assigning a profile won't change the numbers in the file, but it will change the meaning (color) of those numbers. Your current numbers "mean" what the old profile says, so assigning that profile doesn't really change their meaning (color). Converting to the new profile will change the numbers without changing their meaning (color) for the new print condition. When you print with the new conditions, you'll get the meaning (color) intended for the new profile, whether that is attached or not.

Of course, I don't claim to understand color management, so this may be completely wrong.

Preston Earle
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   Date: Thu, 15 Jan 2004 15:42:42 -0500
   From: Terry Wyse
Subject: Re: Converting CMYK to CMYK

On Jan 15, 2004, at 1:19 PM, Cliff White wrote:

So, I need to reconvert the images.  What is the quickest and easiest
way of doing this?  The current CMYK images are untagged.  I have the
original RAW files, but no RGB file with my color corrections and
adjustments, etc.  In the interest of time, I don't want to go back and
redo all of them, so can I simply reconvert the CMYK's?  If so, how do  I
do this?  Open up the file and use "Convert to Profile" to assign the
new target?

Assigning the new CMYK profile will only show you what the current CMYK images will look like under the new target profile/press conditions. If the first profile assumed too much dot gain, your current images will appear too LIGHT (not enough gain) under the new profile assignment. At any rate, this is not what you want to do...

...what you want to do is ASSIGN the OLD profile (the one used for the original RGB->CMYK conversion) to all your existing images and then CONVERT to the new profile. Use relative colorimetric w/BPC for this conversion regardless of the rendering intent you used the first time. The only "gotcha" in doing all this is that all your original CMYK values get lost on their way through this conversion. Things like black-only drop shadows and the like will come out as a "rich black" CMYK mix. The only way to get around that is to use device link profiles and specialized conversion software or simply manually edit the images using curves to adjust the dot gain (if that's the ONLY difference between the two profiles).

That's the basics though.

Regards,
Terry
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WyseConsul
Color Management Consulting
704.843.0858
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   Date: Thu, 15 Jan 2004 13:51:45 -0700
   From: Chris Murphy
Subject: Re: Converting CMYK to CMYK

On Jan 15, 2004, at 11:19 AM, Cliff White wrote:

If so, how do I do this?  Open up the file and use "Convert to Profile" to assign the
new target?  Or do I need to convert back to RGB, before converting to
the new CMYK target, and if so, will that degrade image quality or screw
up the color balance somehow? I'm a little fuzzy on this, so any help is
appreciated.

1. Assign the profile you used to convert them into CMYK in the first place, i.e. the "bad" one.
2. Use Convert to Profile and select the new "good" profile.
3. Convert with relative colorimetric and black point compensation on.

Chris Murphy
Color Remedies (TM)
www.colorremedies.com/realworldcolor
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   Date: Fri, 16 Jan 2004 14:35:50 -0000
   From: Stephen Marsh
Subject: Re: Converting CMYK to CMYK

Terry Wyse writes:

The only "gotcha" in doing all this is that all your original CMYK
values get lost on their way through this conversion. Things like
black-only drop shadows and the like will come out as a "rich black"
CMYK mix. The only way to get around that is to use device link
profiles and specialized conversion software or simply manually edit
the images using curves to adjust the dot gain (if that's the ONLY
difference between the two profiles).

These can be major concerns for some users, and if significant post separation edits to neutral tones have taken place without reference to the separation profile that will be tagged to the image for re-conversion, then the new CMYK mix will not be neutral for these once neutral tones (as neutral CMYK numbers should equate to 0A 0B LAB values when the correct profile is assigned to describe the appearance of the files CMYK numbers).

Sometimes one can simply Apply Image the original K plate to the new seps K plate, or selected portions of the original using Darken blending (one can lighten the midtones so that only darker source tones are applied or to use duped layers in the target and blend if sliders or masks or selections). The source K channel can also be used as a mask to reduce/remove CMY in the K only areas.

It would be nice if DLP's were more common and if Photoshop had the ability to use them (I have only used them in Creo scanner software).

Regards,

Stephen Marsh.

Adobe Photoshop training classes are taught in the US by Sterling Ledet & Associates, Inc.