UNDERSTANDING VIDEO PROJECTOR LENS SHIFT
The normal relationship between a video projector and projection screen is with the lens perpendicular to the center of the screen. Accordingly, if you were to look at a video projector and a projection screen attached to a wall in a side view, a line drawn between the lens and screen parallel to the floor would bisect the screen's height. This is called "on axis" projection or, a lens shift ratio of 1:1 because there is the same amount of image height above the line (Top) as below (Bottom).

Sometimes it is not possible to position the video projector so that its lens is perpendicular to the center of the screen. Tilting the projector will reposition the image, but it will also induce keystoning, which is usually not a desireable result. The objective of lens shift is to minimize/eliminate the need to tilt the video projector to position the image on the screen, by moving the image on the screen. We do this by either moving or fixing the lens off axis in the projector.

In order to understand, we describe lens shift in terms of its effect on the screen, as a ratio, as something you can easily use. We express the range of lens shift available as a ratio of the maximum amount of image above (Top) and below (Bottom) an imaginary line drawn between the lens and screen parallel to the floor. And, if the lens can move both up and down relative to the on-axis ("center") position, we show two ratios (T:B~B:T).

View a detailed image of the Video Projector Lens Shift Diagram or click on the image to the left.

Example: On a 9x12 screen, a projector with an 8:1 (T:B) lens shift offset ratio will be positioned 1 ft above the bottom edge of the image in normal orientation, or 1 ft. below the top edge of the image in inverted orientation.