Originally Posted by dvaught
A little more about CMYK vs RGB
CMYK is an additive method of color theory. The more of each color you push, the darker the color. 0% of all 4 colors = white. 100% of all 4 colors is as dark as you can get.
RGB is a subtractive method of color theory. The more of each color you push, the lighter the color. 0% of all 3 colors = black. 100% of all 3 colors = white.
The 2 theories are polar opposite of one another. One deals with mixing ink, the other deals with mixing light. Digital cameras and most scans are native RGB files because they are produced by capturing light. Working on files in RGB offers a tremendous amount of flexibility over CMYK. It will save time, effort and file size. Working color in RGB is never going to be as precise as working color in CMYK until we begin to print in RGB, which can’t happen unless we begin to print with light because no mater how you try, RGB does not work as an additive method. You can not mix Red, Green and Blue inks to give you a usable color spectrum. Mix those 3 as light and you have an infinite number of colors. So as soon as we start printing with light instead of Cyan, Magenta, Yellow and Black our files have to be converted to 4-color. If you don’t do it, the rip will do it for you. If you convert to a profile, you are trusting the computer to mix the colors you want. In most cases it does a nice job and looks on screen and even on a Fuji/Kodak very similar to your RGB file. I just don't think that profiles are quite there yet. It is getting close, just not there yet. I look forward to the day when converting to a profile works everytime though, it will make my life a lot easier.
NOTE: What you are about to read is where having knowledge of the printing process comes in handy. Here is an example of a skin tone color taken from RGB and converted to SWOP v2. The conversion split the RGB channels into 15c, 40m, 54y, 0k. The IDENTICAL color can be made by altering the color to a 5c, 35m, 50y, 9k. Try it, mix 2 colors and put one on top of another, go through the channel to confirm. Why would you go through that trouble you ask? Because you are spreading the color information across all channels more evenly to lessen color shifts made you color moves made on press. Knowing that pages run on press forms and are subject to color shifts caused by pages running up on the press form it is in our best interest to mix the colors in a manner that allows the most flexibility. Pull a little cyan ON PRESS from the RGB conversion mix of 15c, 40m, 54y, 0k and the color gets real red in a hurry. Push cyan and it goes green. Equally unfortunate results occur from pushing and pulling the colors. BUT.... Wait, your file looked good on the Fuji, why would you need to push or pull any color on press? Because a Fuji shows you what the press is capable of printing, not what it is going to print when you fire up the press. So the press gets running and your image looks too cool. The pressmen tweak on color for a bit trying to match your Fuji but every move they make effects what is running below on the press sheet. In an ideal world you run the same pages up with one another so the color moves made do not negatively effect one another. The unfortunate reality is that is rarely the case. So when you press check you are basically making compromises. Would you rather this image be right on with the Fuji and the one below look greenish? Or do you split the middle and get both close? That is up the the person doing the OK. So if you think past the Fuji proof and set your files up for press you are giving the pressmen more flexibility to match color on press. OR just turn over your RGB files to pre-press and let them dick with your color. Personally, I don't trust pre-press operators to make those moves so I chose to do it myself.
I believe I am a pretty competent retoucher, not the best in my studio and likely not the best of those reading this, but I hold down a day job. I can say that in 15 years I have been fortunate enough to have done thousands of press OK’s and had to make the compromises on press which has given me a lot of experience with the print process and understand how it directly relates to what we do. I certainly don't know everything, but knowing that I can't part the sea or walk on water keeps me open to trying new things.
My job is to make images print to their maximum potential. My studio is fortunate enough to get to work with some fantastic photographers and clients, but we also have to deal with garbage images from time to time. Regardless of the quality of the image, the basic color theory applies to all.