What’s a halftone?
When you look really close at a picture printed in a magazine, you’ll notice that it’s made up of little dots. The reason for this is that a printing press can’t print a lighter shade of an ink – it’s either there or it isn’t. So they simulate lighter shades by breaking the picture up into little dots; this is called a halftone. A white area will have virtually no dots or perhaps dots that cover only a few percent of the area they sit in. A grey area will have dots that are somewhat larger; dark grey larger still, and by the time we get to black, the dots have joined into solid ink.
Lines per inch
If you take a ruler and lay it against a halftone, you’ll be able to count how many dots fit into an inch. This measurement is called Lines Per Inch (LPI) or Dots Per Inch (DPI); it’s often referred to as screen ruling. The more dots you have in an inch, the finer the picture, and the better detail it can show. Images printed on soft, porous paper like newsprint will use fairly coarse screen rulings, often around 85 lpi; on glossy paper like you’d see in a magazine, these dots are finer – typically from 133 to 150 or even 200 lines per inch.
What It All Means
This matters because the pixel density in a picture submitted to a designer has a direct relationship to the screen ruling the picture is destined for. As a rule of thumb, the pixels per inch should be about twice the screen ruling of the printed picture. Since quality printing usually uses a screen ruling of 150 lines per inch, a picture with a pixel density of 300 pixels per inch is pretty standard. If the image has less than that, it will look soft and will display jaggy, rough edges, especially on diagonal parts of the picture, and fine detail will not show very well.
When you use a digital camera, make sure you shoot with the highest image resolution the camera can provide. A designer can always make the picture smaller if he has to – but if he has to enlarge it, image quality suffers.
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