Many designers who are new to preparing artwork for CMYK often make the mistake of setting the CMY (cyan, magenta, yellow) values at 0%, and the K (black) value at 100%. This is not sufficient ink coverage to offer a true 'Rich Black'.
For a dark, rich black, you must include amounts of CMY to build up a base layer of pigment for the black to appear 'true'. Otherwise, with 100K black only, you get a sort of dark gray, or 'milky' black values.
Setting type and thin detailed black:
When setting small type in black on white, 100K black is preferred because the eye does not notice the lighter shade, and there are no other colors to present issues with slight registration misalignments.
That being said, with large areas of black, and bold graphics, you must use a rich black to ensure solid dark print. We recommend a standard rich black value of 60/40/40/100. You will find other combinations suggested when searching rich black on the web, but almost anything more than 0/0/0/100 will provide better results than 100K.
Too Much Ink!!! Registration Black
If you use 100/100/100/100 you're in for trouble, as this is too much ink coverage and you will more than definitely smear / lose detail. A good rule of thumb is that you do not exceed 240 when adding up all of your values. The default for photoshop #00000 black is actually 75 / 68 / 67 / 90, which adds up to 300, and is usually pretty reliable, but for tiny details in large blocks of black ,you may have some issues with clarity.
Below is an example of how the two kinds of black appear printed next to each other, even if you can't tell it from the preview you see in photoshop / illustrator.
Make sure all your blacks match!
It can happen where you are designing something and don't realize it but the black values of different pieces of your design do not match. This will be noticeable when you inspect the printed design closely, as the 'richer' values will appear darker, maybe not at first glance but upon closer inspection. Your computer monitor is not as reliable as the color picker tool. As a designer you must be aware that your black values should match unless you intentionally are setting different values for effect.
For more information on rich black, check out the Wikipedia Article on the subject
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