A few times in the last couple of weeks I've been on forums and noticed people who were (apparently) quite unaware or indifferent about the risks involved in CMYK conversion.
For photographers, CMYK isn't something that comes up very often. And that's a good thing. But now and then you'll be preparing photos for a magazine, or getting your business cards printed, or perhaps being featured in a newspaper. In those circumstances, you might be asked to provide CMYK files.
(Note: Elements users can't convert to CMYK. You'll just have to explain this, and submit RGB files instead.)
Too many people think that CMYK conversion is as simple as Image > Mode > CMYK. Yes, that converts to CMYK, but it converts to the default CMYK profile that's set in your Color Settings (usually SWOP Coated v2). And if that default CMYK profile isn't the correct one for your purpose, you'd better get a bucket ready, because you're going to vomit when you see the printed result.
Everybody knows that there are a few "flavours" of RGB, right? The most common one is sRGB, and the other major ones are Adobe RGB and ProPhoto RGB. And a lot of you will be familiar with the problems that can arise when you use the wrong RGB space - for example, if you post an Adobe RGB or ProPhoto RGB file to the web, it will look awful and dull.
Well, that problem can be much more serious for CMYK, because there are many more CMYK profiles in the world. Heck, I've made a few hundred myself. The result of choosing the wrong profile could be as benign as a print that's a little too dark or too light; or as severe as major colour casts, with people as yellow as the Simpsons.
Don't be alarmed - be forearmed. If you are required to provide CMYK files, ask as many questions as you can. Some printers might tell you that the default SWOP profile in your Photoshop is the right one, in which case you've got nothing to worry about. Other printers might specify one of the other Photoshop profiles, and some printers might supply you with their own custom one.
I wrote an article here about newspaper CMYK, if you need it.
If you're worried about a CMYK job, please don't hesitate to drop me a line at "Ask Damien", and I'll help out.