In practice, image quality can be a confusing topic because the word resolution is used by computer and associated hardware manufacturers to describe the capacity of a display. Photographers and printers use it to describe image quality while, for scanners, it denotes the pixel density of the scan.
Resolution defined: In optical terms, dictionaries define resolution as the smallest measurable interval or the degree of detail in an image.
Graphics Cards and Monitors
The two major components that determine how things are displayed on your monitor are the graphics card and the monitor itself. The graphics card lives inside your computer and processes display information to send to the monitor. The monitor accepts this data and further processes it to write to the screen. Graphics cards generally permit the choice of a number of display resolutions e.g. 800 x 600 or 1280 x 1024. These display resolutions allow you to customize the display size to suit your personal viewing preference - smaller numbers will make things appear larger on the screen. The monitor may not be able to display all of the resolutions available on the graphics card. The highest available resolution is usually specified in the product description.
The key point to remember is that a monitor has an nominal resolution of 72 pixels per inch (ppi)1. This means that an image 72 pixels wide and 72 pixels high will measure 1 inch on the screen when the display resolution is the same as the actual pixel size of the monitor. The 17" monitor I’m using at the moment has a measured size of 960 by 790 pixels (although it is capable of 1280x1024). This display resolution is not available in my display preferences, and anyway I prefer to use the full capacity of the graphics card and monitor combination which, in this case, is 1280 x 1024. This means that an image at 72ppi will display at 0.8 inches (960/1280).
This orange square (above) is 72 pixels wide. The actual size at which it appears on your screen will depend on the graphics card, the monitor and the level of zoom you have set in your browser. On my monitor it displays at about 20mm or 3/4 inch.
Actual Screen Sizes
Depending on the actual size of your monitor, the display card and the monitor itself will have to do some work to fit the image onto the screen. Trying to display 1280 pixels of image on 960 available pixels will result in some loss of pixels and, consequently, some loss of information. If my display preferences were set to 800 x 600, then fitting 800 pixels into 960 available pixels would leave some gaps to be filled.
The graphics card and monitor deal with this by calculating average colours for each pixel based on the colors of adjacent pixels. This process is called interpolation and can account for the fuzzy look you can get when the display resolution is a bad match for the actual size of the monitor. Generally, though, if the display size is greater than the actual screen size, the resulting reduction makes up for the loss of sharpness in the same way that a poor quality photo looks better at arms length.
The square on the left is 2 pixels x 2 pixels (greatly magnified)
The square on the right is the same 2 pixel image enlarged to fit into 3 pixels. The lighter orange area has been interpolated. The black lines have been added to show the original pixel boundaries.
How does this affect image quality?
I find that to achieve the optimum image quality on screen it is best in, say, Photoshop, to resize an image to 72 pixels resolution and adjust the dimensions of the image to the size at which you wish to view it. After changing the size this way it can be beneficial, especially with photographs to use a little unsharp mask. This will have the effect of tightening up any interpolated edges.
- The number of light emitting points in a given distance varies with different monitors and screen sizes, but it is generally accepted that the nominal ppi for computer monitors is 72.