Preparing a CMYK File for Commercial Reproduction
We now move from the realm of personal printers to the world of
commercial prepress. The prepress process involves outputting film
that is burned to metal plates (or output directly from computer to
plates), which are then loaded onto a high-capacity printing press
for mass reproduction. (Hence the pre in prepress.) Each page of
a full-color document must be separated onto four plates, one for
each of the four process colors: cyan, magenta, yellow, and black. By
converting an image to CMYK in advance, you perform the separa-
tion in advance. All the page-layout or other print application has
to do is send each color channel to a different plate, and the image
is ready to output.
Converting an image from RGB to CMYK in Photoshop is as sim-
ple as choosing Image→Mode→CMYK Color. This one command
separates a three-channel image into the four process colors. For
insight into how this happens, read the sidebar “Why (and How)
Three Channels Become Four,” on page 492. Converting the image
properly, however, is another matter. If you don’t take time to char-
acterize the output device—that is, explain to Photoshop how the
prepress device renders cyan, magenta, yellow, and black inks—the
colors you see on screen will not match those in the final output.
Photoshop gives you two ways to characterize a CMYK device:- Ask your commercial print shop if they have a ColorSync or
ICM profile for the press they’ll be using to output your docu-
ment. (They might also call it an ICC profile—same diff for our
purposes.) Such profiles are entirely cross-platform, so the same
file will work on either the PC or the Mac. Just make sure that
the filename ends with the three-letter extension .icc or .icm.
You can then load the profile into Photoshop. - Wing it. Every print house should be able to profile its presses.
But you’d be surprised how many won’t have the vaguest idea
what you’re talking about when you request a profile or will
proffer an excuse about why profiling isn’t an option. In this
case, you’ll have to create a profile on your own through trial
and error. It’s not the optimal solution, but I have done it more
often than not and generally manage to arrive at moderately
predictable output.
432 Lesson 12: Print and Web Output