What is resolution?

There are two related, but slightly different definitions of resolution.

1. There is image resolution, which is explained here: https://guides.lib.umich.edu/c.php?g=282942&p=1885350
2. And there is screen resolution for a tv or monitor.

When talking about image resolution, the number is given as PPI, or pixels per inch. The resolution is not connected to the total height and width of the image, rather the density of the pixels. Therefore, the size of the image doesn’t impact the quality of the image. If an image has 300 PPI resolution, a 5-inch x 5-inch picture will have the same quality as a 50-inch x 30-inch poster.

Screen resolution is a little different. With TVs for example, there are standard “resolutions” like 1,920×1,080 for 1080p HD, or 1,280×720 or 720p HD. When talking about screens, the numbers 1,080 and 720 specify how many pixels there are in the screen from bottom to top. So that means, when it comes to screen resolution, the resolution has nothing to do with the size of the screen’s width and height, just how the screen is evenly divided into pixels. Therefore, in contrast to image resolution, the size of the screen has a direct impact on the quality of the image. For example, let’s say an iPhone and movie theater screen are both 1080p. Since the screen of an iPhone is so much smaller compared to a theater, but the number of pixels from top to bottom is exactly the same, the size of each pixel on the iPhone will be much smaller than that in the movie theatre. Therefore, the PPI of the iPhone is much much higher than that of the theater, so the picture quality of the iPhone will be much higher.