Commercial Printing presents a special set of problems with respect to preservation of color. Since the color model is different, an important conversion must happen to the color information. There's also a collaboration and workflow component that's important to understand.
What is CMYK?
How to choose the correct CMYK profile
CMYK conversion basics
Softproofing workflow for CMYK conversions
Sharpening for output
Convert to profile
CMYK guide prints
Delivering CMYK files
The rest of the story
Cyan, Magenta, Yellow and Black are the colors used in the vast majority of commercial color printing – books, magazines, brochures and more. CMYK color is also commonly referred to as process color. It is a subtractive color model using cyan, magenta, yellow and black inks in color printing. Although theoretically all colors can be produced by cyan, magenta and yellow, printing inks are not totally pure in color and so a black plate is necessary to achieve a true black. As a matter of fact the "K" in CMYK comes from the word "key", meaning that the cyan, magenta and yellow plates are "keyed" or matched up to the black plate.
|Figure 1 In CMYK color, each color is broken out into a channel that represents an ink color. Here's what an image looks like when it's shown in the individual colors.|
Should photographers worry about converting to CMYK?
The majority of photographers shy away from providing CMYK files. Perhaps it’s because many of them remember back to the days of film, making color separations as an arcane process requiring $50,000 drum scanners and specialized RIP software. Meanwhile, many graphic designers blithely push the “convert to profile” button without a second thought. The truth is somewhere inbetween.
Good RGB to CMYK conversion is both an art and a science, but it is not rocket science. We highly recommend Rick McCleary’s CMYK 2.0 as a good guide to this process, the Adobe white paper by Jeff Schewe as well as Digital Photography Best Practices and Workflow Handbook.
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When you make a CMYK conversion from an RGB image, you need to decide which profile to convert to. This can be a confusing choice, and there is a lot of lore out there that's not correct. Let's look at the possibilities.
If the printers can send you a profile for their press, then that's the best one to convert to.
More often, however, the printer will be unknown, or they will suggest that you use a generic CMYK profile, and often they will suggest the U.S. Web Coated SWOP v2 profile that ships with Photoshop. We recommend that you don't choose this as a generic profile, but choose instead one of the following profiles:
- GRACoL is the recommended profile for images going out for sheetfed reproduction. (Sheetfed presses are common for brochures and other custom print jobs.)
- We recommend SWOP 3 or SWOP 5 for web press. Web presses are commonly used for magazines and other high-volume printing.
- If the images will be printed in Europe, then you will probably want to choose one of the FOGRA CMYK profiles. There are two coated FOGRA profiles and one uncoated FOGRA profile that ship with CS4.
When you convert to CMYK, it's important to understand the following points and keep them in mind as you examine the conversion that you produce.
- You are transforming between color models.
- You will be mapping colors to a smaller gamut space.
- Your goal is to preserve as much of the color appearance as possible in the transformation.
- You have more tools to control the nature of the color conversion, such as different rendering intents, because RGB to CMYK conversions use the more sophisticated LUT based profiles.
Here's a basic list of steps we recommend for the creation of a CMYK conversion. The movie will demonstrate each of these and show how using the options can help make a better final image. Additional information on using Photoshop tools to improve CMYK conversions can be found in Rick McCleary's book or the Digital Photography Best Practices and Workflow Handbook.
- Make a duplicate copy of your image file. This will serve as your RGB reference file. Arrange the reference file and the target file side-by-side on your monitor.
- Select main file.
- Open Soft Proofing and select the destination space (the CMYK target space).
- Confirm that the Black Point Compensation and Black Ink boxes are checked.
- Toggle rendering intents to compare the reference file to see which produces the best conversion.
- If problem areas show up in conversion, check the gamut warning.
- If adjustment is needed, create an HSL adjustment layer called CMYK conversion.
- First try to adjust Saturation for affected colors.
- Adjust Hue if more work is necessary.
- Once you are happy with the softproofed target file, it is ready for conversion to CMYK. If you plan to deliver sized and sharpened CMYK, you'll need to do the sharpening workflow step before actually converting to CMYK.
|Figure 2 This video shows how to convert a file from RGB to CMYK.|
- If you know what size the image will be printed, resize the image before sharpening.
- If you don’t know what the final size is, you'll have to choose between sharpening generically or not sharpening, then clearly communicating to the image receiver that the file has not been sharpened.
Make sure that the Convert to Profile settings used exactly match the soft proofing settings used. The important settings are: the target profile, rendering intent and black point compensation. We suggest that you append the letters CMYK to the filename to indicate that the file has been converted to the CMYK color mode.
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|Figure 3 Once you've gone through the soft-proofing process, you'll want to make the conversion and save as a separate file. Here's a movie showing how.|
CMYK guide prints can be a very useful tool especially when the printer is unknown. These are easily produced on desktop printers. The main criteria is that you have a good profile for the printer/paper combination. You should also make the print from either the CMYK proof space (RGB file with the appropriate CMYK profile set as the destination space) or from the CMYK derivative file. Doing either will restrict the colors to the target CMYK color gamut which gives a realistic preview of how the image file will print on the offset press. Using the printer driver will give a good visual match; using a Raster Image Processor will get you even closer, especially if the RIP has a linearization function. Linearization calibrates the printer and makes the profile even more accurate. RIP-driven ink-jets are, in fact, what printers use nowadays for making proofs.
|Figure 4 In Photoshop CS4 and 5, here are the print settings for creating a cross-rendered guide print – one that only uses colors from the destination color space as set up in soft proofing. This ensures that no colors show up in the guide print that are outside the color space of the CMYK process.|
It is ideal to deliver CMYK files with their accompanying CMYK guide prints. However, electronic delivery of final files is becoming more and more common due to speed, convenience and cost considerations. For this reason, we recommend that you deliver CMYK files with an embedded CMYK profile. The printer may well convert your CMYK profile to his CMYK profile, or he may need to alter certain variables such as the total ink limits to match the paper. If he has a known starting point of an embedded profile, his job will be easier and your chances of seeing correct color will increase. For those photographers who are told to deliver RGB files only, one work-around is to actually convert to CMYK — allowing you to control the process and color — and then convert back to RGB. When the printer gets those files and converts them to CMYK, you will have a better chance of having your images appear as intended.
Read more about File Delivery in the Delivery section
Offset presses are different to other printing devices in that they have much greater calibration control at the point of printing. Newer presses are controlled by computers that can adjust the mix of inks at many points across the press sheet with a high degree of accuracy. Just as we gray-balance monitors and gray-balance cameras, good press operators gray-balance their presses. The gray-balancing methodology outlined in the GRACoL/SWOP specifications has the huge benefit of making specific CMYK profiles largely irrelevant except in the unusual case of non-standard ink colors.
Three profiles, one for sheetfed presses and two for web presses, cover the range of papers from premium coated #1 sheets to wood pulp based #5 sheets. Adobe’s version of these three profiles ships with CS4. Or if you prefer, the GRACoL/SWOP versions can be downloaded from the SWOP.org website. The GRACoL Coated profiles can be used in place of U.S. Sheetfed Coated and The Web Coated SWOP 2006 Grade 3 or Grade 5 paper profiles can replace the U.S. Web Coated v2 profiles that shipped with earlier versions of Photoshop up to CS3.
We have discovered that even non color-managed print shops print better from the new GRACoL or SWOP profiles (as long as they don’t assign a different profile to the image files). If you are provided with a specific CMYK profile, by all means use it; however, don’t assume that just because the printer is unknown you are unable to deliver good CMYK files. An additional benefit of the GRACoL/SWOP methodology and the accompanying CMYK profiles is that they maximize the CMYK gamut, getting the most color out of the press and making conversion from RGB easier, since less color gets clipped. Although the new profiles are all formulated for coated stock and have correspondingly high Total Ink Limits, most printers can easily bring those values down for uncoated stock via Photoshop, a device link profile or by means of the RIP.