Quality loss and harsh color when RGB converts to CMYK
Tony Bamonte from Spokane, Washington, USA, using Adobe Photoshop CS (8) with IBM, writes in with:
[Quote:] I am scanning vintage postcards for a book project. When I convert my scans from RGB's to CMYK's, which the printing industry demands, I lose the quality. They become harsh, unnatural looking and slighly blurry. They look great as RGB's. What can I do, or how can I adjust them to look their best. I'm scanning at 300 dpi. Thank You.
Guess what: CMYK cannot reproduce all the colors you find in RGB. Those unmatched colors are referred to as "Out of gamut" -- therefore when you convert from RGB to CMYK, Photoshop automatically modifies all the colors in the file to their nearest CMYK values -- which can cause the troubles you're experiencing.
Ideally, your best bet is to actually scan in CMYK. See if your scanner has that option. If you do a lot of this kind of work you would be well served to purchase a scanner (or scanning software) with CMYK capabilities. If you have just a few to scan, check the local service bureau or photo store. If they can scan it for you, they usually don't charge much.
One way to attempt a closer color match is to use the Custom Proof setup.
Choose: View > Proof Setup > Custom
You can toggle off and on the "Preview" button to see before and after.
Experiment with the Profile and Intent pull-down menus to try different settings.
If you are unsuccessful, then Choose: View > Gamut Warning
The colors indicated are those which will be affected. If they are simple and appear not to influence the entire file, you can attempt correcting them using the Sponge tool (Tap O) with Desaturate from the Mode pop-up menu.
Now begin attempting to pull the colors back into gamut by lightly painting over the out-of-gamut colors with the sponge tool. As they come into gamut their color will return. If desaturation is too severe, the colors will go dull, you must be careful.
Select View > Gamut Warning again to re-proof the color.
You can also make a SELECTION of all out-of-gamut colors using Select > Color Range > Out-of-gamut. Using this will select those color areas to restrict your editing to just the selections.
Ideally, your best bet is to actually scan in CMYK. See if your scanner has that option.
Bottom line: RGB suffers a loss of color when converted to CMYK. Use the above to get "close" to a desirable image.