The exact center of a square or rectangular movie screen can be difficult to determine. When attempting to line up a projector, for example, the true center of the screen is required for proper presentation. It is also useful when calibrating a computer monitor or television for horizontal and vertical positioning.
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Measure the diagonal dimension of the screen starting with the top left corner and measure to the bottom right corner. Repeat this for the top right corner down to the bottom left corner. These dimensions should be equal.
Measure two pieces of string based on the diagonal measurements, then add two inches before cutting to length.
Tape one end of one string to the top left corner of the screen. Stretch the string to the bottom right corner and tape it securely. Ensure both strings are exactly on the corners. Repeat this with the second string from the top right to the bottom left. The point in the middle of the screen where the two strings crosses is the exact center of the screen.