What Exactly is PPI or DPI?
I’ve been creating digital content now for almost 15 years and it remains to be frustrating how confused people are about the meaning of pixels (or dots) per inch. Either it be working for a web client, a magazine editor or with another photographer, the reference to the resolution of an image file in terms of pixels per inch continues to surface. What exactly does 72 pixels per inch mean? Nothing at all on it’s own because without a physical print size, that could literally mean anything.
DPI (or PPI) is the value that tells the printer how many drops of ink to place on a page for every inch of paper but it has very little to do with the actual files resolution. From a computers perspective, the only values that matter are the total number of pixels and a 4×6 inch print at 300dpi is the exact same file resolution as a 16.5×25 inch print at 72dpi. Note that I’m talking about the file resolution and not the print resolution. In both cases, the file itself has 1800×1200 pixels.
300 pixels per inch * 6 inches of paper = 1800 total pixels.
72 pixels per inch * 25 inches of paper = 1800 total pixels.
In my opinion, the dpi (or ppi) is simply the printed results of the true resolution. If someone asks for an 8×10 print at 260dpi, what they are really asking for is a resolution of 2080×2600 pixels. This same file would print 16×20 at 130dpi proving that without a width and height measurement, the dpi value by itself is simply a random number. And you can’t define a width and height using inches on digital displays.
Which brings us to the myth claiming the internet is 72ppi. It’s not true because no browser actually reads the ppi value and simply displays the image at 100% in relation to the grid of pixels known as your monitors resolution. That monitor resolution is a fixed value and unlike printing where you can control how many dots of ink (dpi) fall on the page per inch, you can not control the pixels per inch (ppi) of an image displayed on your monitor. If your destination is anything but print, talking dpi just complicates everything. Ignore it.
Unfortunately many popular sites are still teaching that 72 is required but today’s monitors are much more advanced than those original Apple 72ppi monitors. Take this chart for example. The monitor I’m currently typing on is set at 128 pixels per inch. If you are at all curious why 72dpi started in the first place, consider reading this article on text sizes.