If a projector is not perpendicularly aligned with the screen “Keystoning” can occur which means the image looks trapezoidal rather than square. Keystone issues are normally corrected by physically moving a projector left, right, up and down or tilting the projector.
Needs Vertical Keystone Correction
Needs Horizontal Keystone Correction
If you can't move the projector. higher-end projectors offer vertical and horizontal lens shifting to solve alignment issues. Lens shift is the preferred way to correct keystoning because it doesn't affect resolution.
If a projector does not offer lens shift a user can resort to digital keystone correction to try to properly fit the image on the screen. While utilizing Digital Keystone Correction reduces resolution, many users are willing to sacrifice a little clarity to ensure that the projected image properly fits their screen.
Some projectors offer both vertical and horizontal adjustment while others may only offer vertical adjustment. Horizonal keystone correction is designed fix issues caused by the projector not being aligned horizontally (left, right) with the screen. Vertical keystone correction is designed to fix issues caused when the projector is place too high or too low to properly align with the screen.