Quick, go and grab an old 4x6 photo of your childhood. Look really closely and tell me what you see.
But seriously, what do you see?If you look really, really closely with a magnifying glass, you might see something that surprises you: dots! Lots and lots of dots! That’s because, when you get pictures developed and printed, machines print photos by putting thousands of dots on paper to make the image. They’re measured in a thing called DPI or Dots per Inch.
That’s a great question! To get to the bottom of why printers use dots to print images goes all the way back to the days of newspapers. Basically, printers at that time didn’t have a reliable way to reproduce images. Since they could only use black ink on a white page, it was really hard to make grays. Their solution was to use closely concentrated dots to make the eye see gray instead of just black and white. Thus, DPI was invented.
The Addition of Color
It turns out that printing photos with dots is a really efficient way of making colors and adjusting for light tones too. By changing the size and spacing of the dots with a few colors (cyan, magenta, yellow, and black), you can make your eyes see basically any color. By mixing those four colored dots with each other in different spacing, you can make shades, tones, and brightness that’s basically identical to real life.
One of my favorite artists, Roy Liechtenstein, emulated this DPI method to make his paintings. Not only did he use the familiar newspaper comic panel format to construct his paintings, he also used a bunch of dots to make them.
While most home-use printers today don’t use dots to print, many places still do. Newspapers still use dots, as do pharmacy picture printers. And they print lots and lots of dots. DPI varies between different printing media, but they add up to a bunch of dots. Here’s how the DPI compares between different printing media:
Typical 4x6 Photo
If you take these numbers into account, and do a little bit of math, a standard photo has over 3.1 million dots on it! That’s a lot of dots! When it comes down to it, we’re basically always looking at a whole bunch of dots all of the time, but because of the way that our eyes work, we don’t ever notice it. We just see nice, pretty pictures.
To recap, DPI stands for dots per square inch, which is a way to measure how printers print photographs. If you want to calculate DPI, all you do is count the amount of dots that fit into a square inch, and you’ll have it.
So get out your photos, whip out your magnifying glass and get to counting!
Or you can just take our word for it.