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Look at any magazine with color photographs, all of those colors are the result of combinations of cyan, magenta, yellow, and black inks. Those are commonly referred to as the process colors, and I'm sure you've heard the shorthand, CMYK. As the inks are laid down on press, you can see the final color take shape. Different printing plants lay down the four colors in various orders, but the final result is the same. So I want to show you how the piece of paper looks as each successive color is applied.
Here is just the black plate alone, and you can see the range of tones within the black plate, from Shadows to Highlights. Then when I add the cyan, you start to see the image take shape. Now the magenta, and now the yellow, and there is the finished four-color piece. Here's a consideration, on a printing press you cannot print continuous tone as you could in a photograph, you have to print little dots, and that's how the range of tones gets rendered. Here if we look at a half toned image, this is the way ink really hits paper.
You can probably see a little bit of a pattern, and when I zoom in you start to see all the little dots that add up to the four colors. Again, we're going to look at the individual plates. There is the black plate, notice the angle of the lines, notice the angle of the little rows of dots. If we look at the cyan, you'll see it's a different angle, and magenta is at 45 degrees, the yellow is at 90 degrees. Now, different printing plants use different workflows and they may have different angles from the ones I'm showing you here, but the concept is the same.
Because each color prints at a different angle, you have to make sure that there isn't a pattern, that you don't have an unpleasant moire, an interference between the angles of the individual inks. If you have the correct angles and everything meshes nicely, you're going to see what's called a rosette. If you look in the upper right-hand corner here, I think it's easy to see those little rosettes. That's the sign of all the individual colors being at good angles, and you're not going to get an ugly pattern when it's printed. Here in Illustrator, I'm showing you how those little halftone dots fall on a page.
Look, each one is centered in little grid cell, it's sort of like graph paper. The number of cells along the line is called the line screen. If there are 150 little dots in a row in an inch, that's 150 line per inch halftone, and that's a pretty common value for magazines. For a newspaper it might be lower, coarser, 85 lines per inch, or maybe even 65 lines per inch. The finer the line screen, the finer the detail you can hold on press. Very coarse line screens don't hold detail very well, because little bitty details are hardly bigger than knows halftones themselves.
So for something like a coffee table book, it might be above 150 lines per inch, it might even go up to 200 or more. So now you know how little dots of color miraculously render a full-color photograph.
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