In article <efdjpv$gn6$>, "Rale"
wrote:
I have one huge request. I have some pictures in RGB format which I need to convert in CMYK format, because they have to look identical on the screen and printed. Maybe someone can do that for me (convert) or only instruct how I will do that by myself, by which program. Please, send my on email
How to convert from RGB to CMYK:
You can just use Image->Mode->CMYK, and you’ll get a CMYK image.
However, depending on how that image will be printed on press, and what kind of paper is being used, the results may not be printable.
For best results, you MUST use File->Color Settings->CMYK Setup, and use the CMYK Setup dialog box to specify the separation parameters for your press and paper, EVERY time you create a CMYK separation.
Also, many colors in RGB can not be reproduced in CMYK. CMYK has a different range of colors, or "gamut," than RGB.
If your image contains out-of-gamut colors, these colors will be converted to their nearest approximation in CMYK. The result will be a color which is less saturated and somewhat flatter.
Often, a little bit of color tweaking is necessary in the CMYK image. After separating an RGB image to CMYK, you may wish to use the Curves command (Image->Adjust->Curves) to increase contrast in the midtones slightly, as the separation often becomes flatter in the midtones.
Specific colors can be tweaked with Image->Adjust->Selective Color. For example, if your blues have yellow in them, you can remove yellow from blues to make them more saturated and richer.
When you color corrrect the image, you should, of course, have your Info palette open. Look at the numbers in the out-of-gamut colors; see if your primary colors have any contaminating color that can be reduced to increase saturation. For example, yellow in your blues or cyan in your reds can be reduced, if present, to make the colors richer.
Vivid RGB blues often separate with too much magenta, making the colors appear more purple than blue. Using Selective Color to reduce magenta in blues will often solve this problem.
The range and depth of color you can expect to get depends on the settings in your CMYK setup, which themselves depend on the kind of paper and press you are going to be printing on.
As for setting up your CMYK separation:
If you just go Image->Mode->CMYK without changing the values in your CMYK setup, the result will look okay on most sheetfed presses and coated (glossy) color stocks, but will not be acceptable for, say, newspaper printing.
For best results, you should talk to your printer about what to do. Most importantly, make sure he gives you the values he needs for maximum ink and maximum black percentages; if you exceed these values, the image may smear on press.
As starter points:
For sheetfed presses printing on high-quality coated paper, you can usually use GCR, 100% black ink limit, 300% total ink limit, Light or Medium black generation.
For very high quality lithographic output, your total ink limit can go as high as 310%.
For web-fed presses on glossy paper, or sheetfed presses using high-quality uncoated paper at a 110-line halftone or above, use GCR, 100% black ink limit, 280% total ink limit, Light or Medium black generation.
Newsprint is a different story entirely. For newsprint, appropriate settings might be UCR, 240% to 260% total ink limit, 85% black ink limit. When you convert the image to CMYK, it will look washed-out and flat, but newsprint darkens *considerably* on press so this reduction in density is necessary.
Hope that helps.
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