Occasionally, photographers will need to convert an image to CMYK (Cyan, Magenta, Yellow and Black) from RGB (Red, Green, and Blue). This is because most of the printing/publishing world relies on the four-color processes, and Photoshop’s native mode is only three colors.
While you can convert from RGB to CMYK with the click of a mouse button in the Image Mode dialog box, it will produce the same kind of results you get when you use Photoshop’s simple CONVERT TO GRAYSCALE approach to make a color image into a monotone one. In other words, don’t do it.
CMYK conversion is tricky because CMYK is extraordinarily device dependent. In other words, a CMYK print made on five different offset presses will look different each time. Each press has its own characteristics that control how the print will look.
To get the best match, do some tinkering with Photoshop. First of all, nearly all web presses use a 133-line screen. This is equal to 266 DPI. Yet most photographers work in 150-line screen mode or 300 DPI.
If you know you are printing to a web press, just set your scanner to scan for a 133-line screen and you will get better results. You may have already scanned at 150-line screen. If so, go to IMAGE RE-SIZE and select 266 DPI over 300 DPI and rely on Photoshop to make the image smaller. This is not the best choice, but far better than expecting the web press to do it. If you remember some of my earlier tips, it’s always best to scan at the expected print size and resolution. That means sometimes making more than one scan.
Since most of you shoot with digital cameras, this is easier to deal with for you. Just make sure you set the native output resolution of your images to 266 DPI and do so WITHOUT up or downsampling.
The next step is to use the ICC profile for the specific press you will print on. Most reputable printing companies will supply you with their profiles. This is where the term SWOP (Specifications for Web Offset Printing) comes from.
The best-case scenario is that you have these specs BEFORE you work on your picture, so that you can go into Photoshop’s Color SetUp menu to change your SetUp to match the target output profile. Converting what you have on the screen in the case of a profile mismatch is not a good idea because it is just a guessing game. Photoshop will interpolate (guess) which pixels to add or remove.
If you have started with an RGB image, remember that RGB has a wider color gamut than CMYK. Review your work by choosing CMYK preview in the View mode before you go to the printing press.
While there are workarounds that will allow you to use one final edited version of a photo for every situation, some of the workarounds are more trouble than just starting with a new image, new retouching, and a new final print that is perfectly optimized for the press you will print on.
No matter how you capture your image, you still need to convert to the appropriate profile and size your image according to the line screen used by your printer.
This is intended to be a simple primer. There are many ways to convert to CMYK. I wanted to cover the basics for those unfamiliar with the topic. If any of you have web sites, books or other resources you’d like to share on the subject, please do so in the comments section.