When a digital image is displayed on the computer screen, its size is determined by three factors—the screen's resolution setting, the screen's size, and the number of pixels in the image.

The Screen's Resolution

The size of each pixel on the screen is determined by the screen's resolution setting. The resolution is almost always given as a pair of numbers that indicate the screen setting's width and height in pixels. For example, a monitor may be specified as being a low-resolution 640 x 480, a medium resolution of 800 x 600, or a high-resolution of 1024 x 768 or more. (The first number in the pair is the number of pixels across the screen. The second number is the number of rows of pixels down the screen).

Screen Resolution and Image Size

On any given monitor, changing screen resolution changes the number and size of pixels used to display objects such as icons, text, buttons, and images. As shown in the margin illustrations, as the resolution increases, pixels and object sizes decrease making the objects appear sharper.

One way to think about the size of each pixel is in terms of how many pixels are displayed per inch on the screen—pixels per inch (ppi). The larger the pixels, the fewer fit per inch. As you can see from the table on the facing page, the actual number of pixels per inch (the numbers in italics) depends on both the resolution setting and the size of the monitor. (Advertised screen sizes are based on a diagonal measurement. The sizes we're referring to here are horizontal measurements across the screen so they don't relate exactly to advertised screen sizes).

If a 14" monitor and a 21" monitor are both set to 800 x 600 pixels, the pixels per inch are different. On the larger screen the same 800 pixels are spread along a longer row so the pixels per inch decreases. One number you will often see quoted is 72 ppi. This is supposed to be a magic number in digital imaging. Its origins are said to go back to early Apple computer monitors that had that setting. However, it no longer has any meaning except as an approximate average for all monitors. It may as well be 62 or 82. As you can see from the table below, images can be displayed at a variety of ppi—it all depends on the monitor, not the image. The table shows a range of ppi from 30 to 91 but more expensive displays may have even more pixels per inch. It's best to forget the 72 ppi number and think of screen resolutions when sizing images for display on the screen. If their width and height in pixels is less than the screen's resolution, they will be fully displayed. If they are larger, the viewer can only see part of the image at a time and will have to scroll around it—somewhat like reading a newspaper with a magnifying glass. For this reason, most images to be sent by e-mail or posted on a Web site are sized to the lowest possible common denominator—no larger than 600-800 pixels wide or 400–600 pixels high.