Lens Shift and Keystone Correction

Lens Shift

Lens shift is the ability of a projector's lens to move the light rays up, down, left or right. It is accomplished by moving optical parts within the lens. It enables to position the projector outside the optical center of the projected image. Vertical lens shift moves the projected image up and down and horizontal lens shift moves the image left and right. The range of shift varies by projector. Horizontal lens shift is normally not as extensive as vertical shift. Because lens shift is a mechanical operation within the lens, the maximum possible vertical and horizontal lens shift are depending on each other. Vertical lens shift can be used at the top or at the bottom of the image. Normally the projector will be turned upside down if used at the top of the image. Some manufacturers call the situation with the projection axis at the upper or lower image boundary 100%, some manufacturers call the same situation 50% !

25% lens shift 50% lens shift

Keystone Correction

Keystone correction is a different approach to correct image size and position. The keystone correction is a function of the projector's scaler. The projector resizes the image so that the image will appear to become square again. The projector must be tilted such that the image fills the screen. Because of the tilting, a trapezoid image will be created. The keystone correction in the scaler blankes out pixels in the not-used area and rezises the picture out to generate squared image again. The projector's processor actually scales the image in every pixel line to fit to the new right-angled image form. The remaining parts of the image on both sides are 'black'.

Down sides of keystone correction:
- remaining light output could be visible at the blanked-out triangles in a totally dark environment
- loss of image pixels at the 'squeezed' side
- straight lines within the image are not smooth anymore but step like because of the image scaling

projection off axis trapezoid image with corrected (cut-out) image
and distorted vertical line because of image scaling

center projection vertical lens shift keystone distortion and correction

vertical keystone correction horizontal keystone correction vertical and horizontal keystone correction