RGB and CMYK. Two sides of the same coin.

Most photographers use RGB when editing images in Photoshop, but have little idea about whats going on in the CMYK world.

In the days of analog, photographers would submit c-prints, negatives or transparencies to their print shops or Art Directors and it was the responsibility of the printer to make the CMYK separations for their presses. However, in the age of digital, it has become increasingly common for photographers to supply CMYK separated images when their clients request them.

CMYK is only slightly more complicated than RGB because CMY is the inverse of RGB. Both color spaces are directly related. If you remember the words "additive color" and "subtractive color" in that art class you took in college then you have a much better memory than I do.

Very quickly, additive and subtractive color goes something like this. If you add Red in RGB color space, you are adding red to the visible spectrum. In CMYK, if you add Cyan, you are subtracting Red from the spectrum. So, CMY is the inverse of RGB. Cyan subtacts Red. Magenta subtracts Green. Yellow subtracts Blue. Additionally, 100% RGB equals white and 100% CMY equals black.

RGB and CMYK are the same, but just turned upside down.

But what about that fourth letter in CMYK? What's that all about? In printing, 'K' or Black is a futz. A hack. A bailing wire and chewing gum solution. We shouldn't need it at all, but we do, and here are the reasons why.

Ink is printed in percentages of halftone dots. If we print 100% of Cyan, it means there are no halftone dots! So, if I print 100% Cyan, 100% Magenta and 100% Yellow that means all the color plates are "wide open" and we are printing at 300% ink density. The more precise term is Total Area Coverage or TAC for short. TAC300 is a quick way to note this down.

I saw you nodding off but stick with me.

So here is our problem. On thin, inexpensive media, such as newsprint running through web presses (very fast, very large, roll fed printing press), TAC300 is far too much ink for that paper to handle.

In the best case scenario, the image will look dark and muddy and will bleed right into the important news article on the other side. The worst case scenario is that the paper will become saturated with ink and disintegrate on press like a cheap paper towel held under a running faucet.

Either way, it's not pretty.

Newsprint works at TAC240, not TAC300, which is fully 60% less than what our three inks can do. So how are we to print dark colors in an image if we have to screen each color plate back to 80%? As strange as it may sound, the answer is to add another ink! One that is completely neutral and which can help us make our colors darker with a lot less ink.

Lets say we are printing red with 100% Magenta and 100% Yellow on newsprint.This red is already at 200% TAC, but we want the color to be much deeper. Without black, we would have a serious problem because Cyan just isn't that dark. We would need a lot more Cyan to equal the same darkening value as only a little bit of black can do and, as we mentioned before, newsprint won't allow us to use more than TAC240. This is the reason we use black ink in addition to CMY.

Black also comes in handy in all sorts of printing environments. While newsprint can only handle TAC240, other types of presses running coated media can handle a lot more ink, sometimes up to TAC360 or more. We use that extra ink load to create darker and richer and colors than CMY alone could accomplish.

We've discussed how RGB and CMYK are similar and why we add Black to account for different Total Area Coverage (TAC) requirements on press and paper. All of the complicated decisions about CMYK are decided for you when you convert from RGB to CMYK in Photoshop. Specifically, all the decision are being made by the CMYK ICC Profile you are converting into.

There are a few more things about CMYK that I will cover in subsequent posts. For now, I wanted to show the similarities of RGB and CMYK and a few differences as well. If you are not 100% comfortable making your own CMYK separations in Photoshop, request help from your printer about which profiles you should use and always demand contract proofs such as a Kodak Approval from your printer.

Until next time.