When a projector is projected upward or sideways, the picture is distorted as a keystoning. There are two types of keystoning; vertical keystoning where the projector is above or below the centreline of the screen and horizontal keystoning where the projector is to the left or right of the screen.

In general, keystoning occurs when the distance to the top of the image on the screen is much greater than the distance to the bottom of the image. Specifically, if a projector (without keystone correction) is projecting upward at an angle, the image displayed is distorted. Keystoning is caused by the projected image not being perpendicular to the screen, causing the sides of the image to distort and become different widths.

Manual, Digital, Automatic and Fixed Keystone Correction:

Today, there are two methods of keystone correction, manual and digital. Although there is not a standard degree of keystone correction, most screen projectors do offer anywhere between 15 and 30 vertical degrees of correction. The special overhead projectors with 13 ° keystone correction in upward or sideward projection exclude such a distortion up to a projection angle of 13 °. Both models are equipped with a vario-focal lens f = 300 - 340 mm.

Manual keystone correction is essentially a physical adjustment to the lens of the projector so that it projects at an angle higher or lower than it would be if it were on a flat surface.

LCD and DLP™ projectors are both digital display devices. Data such as a PowerPoint presentation sent to a projector, is digitally converted and scaled. By adding a special, selectable, algorithm to the scaling, some new projectors are able to alter the image before it reaches the projection lens. The result is a squared image, even at an angle. This allows presenters more flexibility when setting up their projector in variable environments.

Automatic Keystone Correction function automatically detects real time changes to the vertical angle of the projector during operation and instantly makes the necessary keystone correction to ensure optimum viewing.

Fixed keystone correction is the ability of most projectors to shoot upwards, which makes tilting the projector upward less necessary. If fixed keystone correction is listed on a specification sheet, it will give the angle degree that the image shoots upward.