A method of measuring the size of a screen or a projected image. It measures from one corner to the opposite corner. A 9FT high, 12FT wide, screen has a diagonal of 15FT. Throughout this document we assume that the diagonal dimensions are for the traditional 4:3 ratio of a computer image as per the example above. Some screens are square, others particularly wide for 35mm slides 3:2 ratio. As such even if the screen is 12x12, we would rate it 15FT diagonal since that would be the diagonal of the usable area. OK, how about this! Remember high school? Here's your old geometry lesson. X-squared times Y-squared equals Z-squared. 3ft by 4ft screen = 3 squared (9), + 4 squared (16), equals 25 (5 squared) a 5 ft diagonal image.
The method of measuring the size of a computer or television image. Measured from one corner to the diagonally opposite corner, a 9 FT high by 12 FT wide projected image has a diagonal of 15FT. Typically, in computer and video we assume that the diagonal dimensions are for the traditional 4:3 aspect ratio as in the previous example. Remember, the 4:3 aspect ratio is standard for Video and computer images, but some screens are square, others particularly wide.