The conversion between RGB and CMYK modes is often avoided by PhotoShoppers who are just starting out in color correction.  And no wonder!  The Internet is rife with warnings about the awful things that converting your RGB image to CMYK will do. 

There is a grain of truth to this concern, for graphically created images that contain saturated colors .  For these "artifical" images, converting to and from CMYK is a death knell for bright colors.  But photographs have less saturated colors than graphic images, so "fear of converting" is usually unfounded for those images.

First, let's give the devil his due, and check out the types of problems people run into when converting from RGB to CMYK.  Graphic artists specialize in creating non-photographic, "artificial" images, and they need to know if their work is destined for the RGB web or the CMYK printed page.  In particular, if an

Converting a pure RGB Blue to CMYK  is as bad as it gets.  Check it out:

figure 1

The  good news for photographers (as opposed to graphics artists) is that  99.9% of the time you won't be seeing this kind of blue in a photograph - check out the sky in Example 3.  That's a "real" blue, from a real photograph taken on a dazzling Hawaiian day, and here's what that blue looks like after going to and from CMYK.  

figure 2

So your photographs will not have this conversion problem, and with the examples that follow, I hope to show you that  they can be converted to and from CMYK color space one or more times, with impunity.  

The important example, for you, is the one you will hopefully do yourself using your own images.

Will CMYK mode alter your photographs unacceptably?  

The experiment to answer this question is this: convert your favorite image to CMYK mode and see if you like the way it looks.  Then convert it back again and see if you still like it.  If your answer is yes, then the wide and wonderful :-) world of CMYK color correction is open to you!  

(note: if you are using PhotoShop 6 or later, and if your colors gray out on you in CMYK mode, set your profile intent to Relative Colorimetric instead of Perceptual.  PhotoShop  5 appears to be permanently set to Relative Colorimetric mode.)

More important than the numbers, though, is to look at the large version of each image and see if you can detect any differences between them.

See the color settings used for these examples

Example 1
shadow detail

1 rgb-cmyk cycle

Example 2
color

 image converted using PhotoShop 6

1 rgb-cmyk cycle

Example 3
blue sky!

1 rgb-cmyk cycle

Why use CMYK?  
This is  really a discussion for another web page, and but here are a couple of things that may get you interested.  You may alter cyan, magenta, and yellow without unduly altering objects in your picture that do not contain these colors.  The K channel acts as an anchor that makes it possible to alter the colors of your image very aggressively, without changing the overall brightness of the image, or unduly modifying neutral colors. 

You can directly control shadow detail by changing  the K channel. The K channel is a better place to do your sharpening for two reasons: 1) it contains the important detail of your image, and 2) it cannot introduce artifacts. Finally, CMYK is more intuitive: any kid who has mixed a blue crayon and a yellow one to get green already knows how CMYK colors mix together.

But I want to use RGB to get that blue into my photographs
You still can, if your images are destined for the web, or other RGB medium.  Just correct in RGB as you always did.  If your images are going to be printed, though, you are going to have to compromise on your blues, and there is no better or more correct color space in which to perform that compromise than CMYK.  Note: see my new article, which uses a new CMYK space to address this concern more fully - mgr.

But my printer accepts only RGB data!
This is a valid concern.  Unless you are printing to a Postscript Laser printer, or other CMYK device, PhotoShop will convert your colors back to RGB before sending them to the printer.  Doesn't this undo all the good benefits you had from working in CMYK?  The answer is no - as you can see in the examples, the conversion back to RGB does not significantly degrade the image data.  RGB, with it's wider gamut, is a fine pipeline for sending your CMYK data to the printer.

But wouldn't we like to send CMYK data directly to the printer?
Yes.  If this were possible, we could print an absolutely pure, shining yellow that would make an RGB CRT monitor turn green with envy.  We could also simulate a press more accurately.  But this really has nothing to do with the extra power we already get  by using CMYK as our working color space. 

I Still Don't Buy It, and will continue to color correct in RGB
Well, thanks then for your willingness to read this far.  If you have an RGB photograph that changes when you convert it to CMYK, I'd be interested in seeing it.   Or if you just don't follow my discussion or think I've made a mistake, please email me. 

 If you don't see the benefits of color correction in CMYK, perhaps you will tune in again when I add a web page of examples where color correcting in CMYK gets better images, more easily, than RGB.