The Relationship Between Pixels and Dots
PPI and DPI
The world of printing, like every other field, was turned on its head when home computing became popular; the effects are still felt today. You’re more than likely reading this on a screen right now; move it to another screen and it will stay legible. In fact, if everything is working properly, no matter which screen you view this document on, the scale of everything should hold; graphic elements will be the same size relative to the font, even if their actual size might have changed when you swapped screen. The same should hold true if you print the page off; an element that’s twice the size of another element should remain twice it’s size no matter how you view the document.
This, of course, is not happening by chance; graphic design works on scale. When measuring elements, the standard is to use inches; a bit antiquated in the age of the metric system, but functional nonetheless. You’ll usually hear of two different types of measurements, PPI and DPI. PPI means pixels per inch, while DPI means dots per inch. They’re kind of the same thing, but also kind of different. Let me explain.
Pixels are “picture elements”; they are, in effect, the smallest unit a screen can display. Dots are the smallest thing a printer can print, and that’s why pixels and dots are similar; they’re the smallest possible element. Depending on your screen, you may have 72 or 96 PPI; that means that for every inch you see on your screen, there are either 72 or 96 pixels. Screens can seem like magic, so it’s important to remember that pixels are physical elements. Imagine a row of multicolored lights you can flip on or off individually; that’s roughly analogous to what’s happening when you display an image on your screen. That means that a screen can’t display more than a certain amount of PPI; the bottleneck is physical.
DPI is used exclusively in printing, and it differs from PPI in a key way. While your screen may have a maximum of 96 PPI, pixels refer to the smallest physical element that can be seen by the human eye. Printed images can be anywhere from 150 DPI, for black-and-white newspapers, to 1200 DPI, for extremely high quality prints. The difference is that the printer will use a number of different colored inks in order to create the right color; the more dots it can print per inch, the more accurate the colors will be.
All this to say, don’t get confused between DPI and PPI! When your display is 96 PPI, and you get it printed at 1200 DPI, you won’t end up with a distorted image, and the scale won’t be affected; you’ll just end up with a higher quality print than had you used less DPI. Your print shop will have all kinds of different printers, so you should vary the DPI of the printer you use depending on how high quality you need your prints to be!