Re: rgb to cmyk conversions
Re: rgb to cmyk conversions
- Subject: Re: rgb to cmyk conversions
- From: "Richard Frederickson [Contr]" <email@hidden>
- Date: Sat, 25 Feb 2006 12:10:03 -0500
Title: Re: rgb to cmyk conversions
Welcome to the challenge of Prepress Color!
Is it possible to find out what printing standard they are using?
Ideally, you would want a profile of the press, but that sounds like
it might be hard to come by since you refer to it as a "mystery
press." At least if you knew what standard they use, you'd have a
chance at preparing files with good color. (One of the color gurus on
the list will likely be able to provide insight into this--right
guys!)
There's lots of good books out there, and I'm sure others on the
list will point you to some excellent titles. In the meantime, here's
a simplified overview of the process (assuming Photoshop CS, or newer;
and your color managed workflow)...
Step 1, Preview the color.
While still in RGB color, go to View>Proof
Setup>Custom...
Select your target CMYK color space (Device to Simulate).
Select your Rendering Intent (most likely Perceptual if these are
photographs).
Save this setting and use it when working on the project.
You will notice in your title bar the addition of "/<your
saved setting>". You can toggle the preview on and off with
View>Proof Colors.
Step 2, Adjust your color.
Using Layers...
Set your black point and white point in Levels and check for
clipping.
Set your color correction in Curves.
Adjust contrast in a new Curves layer.
Use Hue/Saturation, Selective Color, and/or Channel Mixer if you
need.
Do NOT use Contrast/Brightness.
[Don Hutcheson has a nice Photoshop Actions file to automate
this. Go to http://www.hutchcolor.com/Utilities.html and download
DonzRGBactions.]
Make sure that your highlights maintain a scum dot of at least
2-3% in two or more of the CMYK channels--except for spectral
highlights, which can be "paper white."
SAVE THIS VERSION (with the embedded RGB profile) AND HOLD ONTO
IT.
Step 3, Convert to CMYK.
Flatten the image.
Do not use Mode>CMYK.
Use Convert to Profile... (this is located under different menus
depending on your version). The default settings should match what you
already set up in the preview (step 1).
Step 4, Tweak the CMYK color.
Some colors, like bright blue, are difficult to achieve in CMYK.
Once your image is in CMYK color space, use Color Mixer in a new
Adjustment Layer to tweak the color further (use the Info Palette to
monitor the effect of your changes).
You can save this version with the embedded profile and your
adjustment layers.
Step 5, Prepare the printer's file.
Flatten your image.
Save as... WITHOUT the embedded profile to the format specified
by your printer.
As you might expect, there is a lot more to this to do it well
(including the use of masks, LAB adjustments, etc.). With some
research, experimentation, and a good dose of trial and error, you
should get the hang of it and come up with a nice product.
Good luck,
Richard
Richard Frederickson
[Contractor]
Senior Technical Specialist
Scientific Publications, Graphics &
Media (SPGM)
SAIC-Frederick, Inc.
National Cancer Institute at
Frederick
Frederick, MD 21702-1201
(301) 846-1546
Visit us at
http://web.ncifcrf.gov/spgm
Hello all,
I am working on a book that will be printed in Hong Kong, and they
want CMYK files and sample prints for color. I have tried some
conversions in Photoshop, but there is a huge difference (to my eyes)
in color.
I am accustomed to submitting RGB files, and I work in a completely
color managed environment. (everything profiled and tagged,
etc...)
Could anyone give me some advice on how to best deal with this, that
is making the conversions to best represent the original files and
have a chance of printing well on this mystery press in HK? Any
good sources on the web or in print? The files do not need to be sent
until August, so I have plenty of time to study and learn.
Thanks in advance,
Peter Calvin
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