Keystone correction

Keystone correction is a projector feature that digitally reshapes the image to fix a trapezoid distortion, where the picture is wider at the top or bottom than it should be. This distortion happens whenever a projector is not mounted exactly perpendicular to the screen, such as tilted upward from a low shelf or placed off to one side of the room rather than centered.

Vertical keystone correction fixes top-to-bottom distortion from tilt, while horizontal keystone correction fixes left-to-right distortion from off-center placement. Both work by digitally trimming and stretching the image to appear rectangular again, but this comes at a real cost: the projector is not using its full native resolution for the visible image, which can soften fine detail compared to a properly squared physical setup.

A common mistake is relying heavily on keystone correction as a substitute for correct mounting, when a small physical adjustment, like a mount with tilt and shift capability or careful shelf placement, preserves far more image sharpness. Keystone correction is best treated as a fine, final adjustment rather than the main method of squaring up the picture.