If you have low vision, these screen magnifiers may help you use both a computer monitor and your television. Depending on the model chosen, these magnifiers fit a range of sizes with different magnification levels. Screen sizes are defined in inches and determined by the diagonal width of the screen. A 21-inch computer or television therefore has a screen that measures 21 inches from top left to bottom right corner of the screen itself, not the monitor or cabinet, which will invariably be larger. Check each description carefully for complete information on magnification levels and screen sizes.