scanning in RGB vs. CMYK: a major issue here!


Dan Margulis <76270.1033@...>
 

Andrea writes,

the CreoScitex local site trainer is saying:
The scanner acquire the files in RGB
The RGB color space has more color info than CMYK
It is fine to acquire the files in CMYK but the CreoScitex trainer
say that it is not necessary>>

All this is approximately correct.

The problem is as follow: several printers and publishers are saying
that they prefer if my company provide them the files as CMYK because
the files are easier to print.>>

All files *must* be CMYK when they get to press, so if you do not provide
CMYK files, the printer must do the extra work of converting from RGB into
CMYK.

Also they do not care to give me the output printer type or paper kind,
so I do not know which profile embed into my files.>>

You don't need to embed any profile. But you do need to know how to make
the conversion between RGB and CMYK. For this it would be helpful if the
printer would give you more information, as otherwise you have to guess at
certain settings.

They keep say that CMYK is much better, because with the K channel I get
images with more vivid colors, more details, better overall.>>

I think that they are saying that if you are an expert color corrector,
CMYK offers certain opportunities to improve the image that aren't present
in RGB.

I am lost! Shall I indeed now go to CMYK or I can use a professional
system like GretagMacbeth color profile and spectrolino to apply the proper
profile to my RGB files?>>

Putting a profile in your RGB file does not solve the problem of how to get
the file into CMYK. It certainly doesn't sound like you should trust your
printer to make this conversion. Rather, you should learn how to configure
Photoshop to do it yourself.

The first question you must ask yourself is whether you will assume a
"generalized" CMYK or whether you are going to attempt to make different
separations for different printing conditions, which is much harder. Most
people assume a generalized CMYK, that is, they find a separation setting
that will create a Matchprint or Cromalin that is fairly accurate. Then
they separate all the files that way, send them to the printer and hope for
the best. This is in fact what you must do if you don't know who the
printer is going to be or what type of press or paper will be used.

To create your own setting, enter CMYK Setup (Photoshop 5.x) or Color
Settings: CMYK: Custom CMYK (Photoshop 6). Choose as your options
Eurostandard (coated), 14% dot gain, Black type: GCR, Black Generation:
Light; Maximum Black=85%; Total Ink=300%; UCA=0%.

With these settings loaded, convert a few files from RGB into CMYK and have
Cromalins made. Now you must judge how they look in comparison to what you
expected. If all look too dark, your dot gain setting was too low and must
be adjusted upward. If all look too light, the dot gain setting must be
reduced. There is also the possibility of adjusting the individual dot gain
curves in the event of color variation. If necessary you now pull a second
set of proofs to confirm your revised settings.

It must be understood that different printers will get different results
from the same CMYK file. If the file is printed on a sheetfed press there
will be less dot gain (a lighter result) than if printed on a web press. If
the file is processed in Italy, where it is customary to image with
positive film, the printing generally will be lighter than it would be in
the United States, where negative film is customary. If you know these
factors in advance, you can adjust for them during the separation. It may
be that you need several different separation settings, if you have
continuing relations with several different printers.

Dan Margulis

Dan Margulis


Andrew Rodney <andrew@...>
 

on 3/28/01 6:54 AM, Dan Margulis at 76270.1033@... wrote:

Putting a profile in your RGB file does not solve the problem of how to get
the file into CMYK.

It helps in we need a description of the RGB to get to CMYK. That was true before ICC profiles (using a monitor preference in versions previous to Photoshop 5) and it’s true today. It’s of no use of she is going to do the conversions (the user will know the recipe of RGB to get to CMYK. Still embedding the profile in the original RGB file does no harm and actually does some good should a conversion need to be made in the future).

If the RGB file is given to another person, having a profile is quite useful. Without it, what assumption should be made about the recipe of RGB for the CMYK conversion? The fact that a good number of printers will get hostile seeing an embedded profile or not know what to do with a file with an embedded profile doesn’t change the facts that a description of RGB is always required to get to CMYK.

It must be understood that different printers will get different results
from the same CMYK file.

That’s an understatement! Yesterday I was showing a student just how varied a certain manufacturer of contract proofs could be from shop to shop. Since I have made dozens of profiles from the same proofing system (but at different printers all over the country), I can use ProfileMaker Pro’s measure tool to look at all 600 spectral readings from patches used to build the profiles and get some very useful stats. In best case, two completely different shops running the same proofing system (Matchprint) were showing an average deltaE of 4 which is quite good. But more often than not, the average deltaE was well over 10 and in some cases double that. Now consider sending the same CMYK file to shops using vastly different proofing systems and the numbers go sky high. A deltaE variance of 6 or less is said to be “acceptable” matching for most clients. Some would reject that much difference in colors. But a deltaE of over 10 means big surprise when that proof does come back!

Andrew Rodney


Chris Murphy <lists@...>
 

The problem is as follow: several printers and publishers are saying
that they prefer if my company provide them the files as CMYK because
the files are easier to print. Also they do not care to give me the
output printer type or paper kind, so I do not know which profile
embed into my files. They keep say that CMYK is much better, because
with the K channel I get images with more vivid colors, more details,
better overall.
Translation of what printer is really saying: "I don't know what I'm
doing when it comes to RGB images and therefore I don't want to be
responsible for making disgusting awful separations. I want you to be
responsible for making disgusting awful separations. I know so little
about desktop color separation I can't even give you a hint as to what
settings to use in Photoshop to make a halfway decent separation for my
printing conditions."

What I would do if you are using Photoshop 6 is use the "U.S. Web Coated
(SWOP) v2" profile if you have no idea how things are going to print.
Otherwise you might try one of the coated or uncoated profiles in that
same list if you know in advance you will be printing to coated or
uncoated stock.

As for embedding profiles, I would recommend staying away from it when
dealing with outside printers who don't know color management let alone
how to make decent separations at the desktop level. An embedded profile
will not help them, so it won't help you. If you like them, use them for
your own purposes and save a no profile version for the printer/service
bureau.



Chris Murphy
Color Remedies (tm)
Boulder, CO
303-415-9932