(Dos & Don'ts)
A printing press or digital printer is not capable of printing actual
shades of gray like in a photo. The printing process fools you into
perceiving shades of gray by using different size dots on the paper.
This is called the "Halftone" process. These dots are really
small so your eye can't really see the individual dots. Instead, your
eye picks up the color of the dot along with the white area of paper
around it and sees it as different shades. The bigger the dot, the
darker shade your eye perceives. In the case of a black and white
photo, the printer only uses black ink. For color printing, the printer
uses Cyan, Magenta, Yellow and Black ink. The "Cyan" halftone
creates the different shades of blue in a photo while the "Magenta",
"Yellow" and "Black" halftones create the different
shades of red, yellow and black. When each halftone is printed and
overlaps another, numerous other colors are created. Areas of "Cyan"
combined with "Yellow" for example, will look green while
areas of "Yellow" and "Magenta" will look orange.
"Screen Ruling" refers to the number of dots there are for
every inch of paper. "Screen Ruling" is therefore often
referred to as "Lines per inch" or LPI. Common "Screen
Rulings" are 150 LPI, 133 LPI, and 120 LPI. In scanning you only
need a resolution equal to twice the screen ruling to get enough information
to create 256 shades. So if your job is going to be printed at 150
LPI, all your scans should be made at a resolution of 300 dpi. For
133 LPI, you only need to scan at 266 LPI. For the most part, however,
people usually just scan their photos at 300 dpi.