Why CMYK?
BBones
Registered Users Posts: 580 Major grins
Recently I have had a couple of magazines using my images and they have been asking me to convert the images to CMYK from RGB. Why? I think that the pictures look "bad" in comparison to the RGB.
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Because CMYK is the language of printers. It gives you the opportunity to optimize the image for printing. It HAS to go through CMYK.
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I have never done prepress work like that, Edgework's the guy to ask.
If you're going to be doing this kind of work, it's most likely important that you understand the theory behind it, and find out what additive and subtractive colors are. The Wikipedia's probably a good place to start.
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James.
http://www.jamesjweg.com
Which way does the Winblows? Every way.
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Correct! RGB is the language of monitors and TVs. CMYK will never read correctly on a monitor (well, never say never). Converting a satisfactory file to CMYK will not hurt the file but it might not look so great on screen. The printed image will look beautiful.
(ps: You should always print a CMYK file on your home printer as well... just consider your printer's ink cartridges)
Moderator of: Location, Location, Location , Mind Your Own Business & Other Cool Shots
Interesting. I have an experiment to do.....
Tim
That's not necessarily true. Most color printers take RGB images even though they print with CMYK cartridges. Assuming you've installed the printer software correctly, and choose correctly when printing, it will perform the necessary interpolations for you.
That's for Desktop printers. For the printer on the corner with the offset press in the back, yeah, CMYK is the way to go.
The reason you should convert yourself is that you need to see what's going to happen to your image when the relatively wide color gamut of RGB gets shoehorned into the cramped confines of CMYK.
All those brillant blues, luminescent greens, vivid lavendars and flaming reds... wave good bye because they aren't coming with you.
Here are some 3d representations of LAB, RGB and CMYK color spaces, in that order. Of course, the LAB representation is being filtered through the RGB space of your monitor, but the comparison is still clear.
You need to decide how you want Photoshop to convert your colors. The two methods that are relevant are Perceptual and Relative, both of which are set in the Color Settings dialogue.
Relative is used in most prepress environments. It converts colors exactly, where possible. (Despite the pathetic range of the CMYK block above compared to RGB, there are a lot more colors in the real world that are reproducable in CMYK than you would think. Think about it: how many day-glo tree leaves have you seen lately? And those flaming reds look awful when they come attached to a face.) Colors that are out of gamut get crushed at the edges. If you have a dress, say, that has many intricate shades of blue to define its detail, you will probably get mud when you convert to CMYK. Most faces and landscapes, and most other things, will convert fine.
If you find that you are losing detail in the conversion, you might want to try Perceptual. That method will reproduce the relationship between all your colors, translated into the new range. You will find that your blue details will show up in the CMYK conversion, but that none of the blues will be the right blue. Nor will any of your other colors. The shift, in most cases, will usually be subtle, and often quite acceptable.
The usual approach is to always use Relative and just wait and see what happens when you convert; if any areas suffer, resort to stealing information from RGB or LAB plates and bring them into the CMYK file as luminosity blends, applied locally with masks. Since we already know that there's no hope of actually printing the original colors no matter what you do, anything you can do to get the image to make sense is agreeable.
That's the bad news.
The good news is that everything your've ever seen in a magazine that blew you away, every stunning poster, every knock-you-off-your-feet album cover... they've all made peace with the limitations of CMYK and produced some brilliant results. So don't give up.
But if you intend to do a lot of print work, don't get too attached to those day-glo greens.
—Korzybski
...until you familiarize yourself with spot colors
No, this is false. Inkjet printer drivers are optimized for RGB files and perform their own RGB>CMYK conversion to the ink colors of the cartridges. Leave inkjet jobs in RGB.
If you convert to CMYK before printing to inkjet, in all probability you have converted based on color specs for printing press inks. Those are very different from the ink chemistry of inkjet cartridges, so converting to CMYK can't help. Particularly if you are printing to one of the 6- or 7-ink inkjets, because those are CcMmYK or CcMmYKk printers. Or consider some Canon inkjets like the i9900, which include Red and Green cartridges to expand the color range. CMYK has no equivalent there.
Because inkjet inks are not printing press inks, many inkjets have a noticeably larger color gamut than printing presses. If you pre-convert (pre-restrict) your image to press CMYK instead of leaving the photo in RGB, you are probably wasting part of the color range of your inkjet.
For logos, backgrounds and special tricks, sure. But it gets a little dicey trying to work a spot channel into an actual photograph. I've seen it done, but only for special effects, like weird colored skin, Martian skies...
—Korzybski
Dan Margulis is an author widely worshipped in these forums for his eye-opening book on editing photos in Lab color space. He originally built his stellar reputation on his CMYK editing knowledge. If you (the original poster) really want to learn some deep tricks for getting images to look their best for a CMYK press, read his book "Professional Photoshop." It is as awesome as his Lab book, and just as challenging. There's so much in that book I can never keep it all in my brain.