It should be a rectangle, but it looks like a trapezoid

Undoubtedly you have seen a problem that plagues many presentations — perhaps you have even battled it yourself — the distorted screen image.

The most common form of this distortion comes as the image loses its right angles. Maybe it is something subtle, just a slightly more narrow width at the bottom of the image than at the top — just enough to make you wonder if your eyes are going. Maybe it is something more severe — a hulking upper half of the image dwarfing the bottom half.

You know what the slide should look like:

But you see something like this:

This common problem is called keystoning, and fortunately, it is easy to correct with the right projector.

What causes keystoning?
Keystoning occurs because of the position of the display device (projector) in relation to the projection surface (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.

Illustrated to the right, the bottom of the image is nearly level with the projector. The top of the image is farther away from the projector.

Of course, the conditions for keystoning occur frequently — whenever the bottom of the image to display is close to "eye level" of the projector. Keystone problems traditionally plague overhead projectors even more than desktop units because overhead projectors are usually placed quite close to the screen.

What is keystone correction?
Keystone correction is simply altering the shape of the projected image to compensate for the distortion caused by having the projector above or below the center of the screen surface. Currently, many multimedia projectors electronically compensate for keystoning and are able to display a rectangular image. Some projectors offer keystone image correction up to +/- 15 degrees, while other projectors offer 30 degree keystone correction. And new state-of-the-art projectors even offer auto keystone correction. This is digital keystone correction that automatically engages — without manual adjustments.

Most projectors use optical techniques (such as lens shift) to preset the keystoning for the projector to be approximately even with the bottom of the screen. (Of course, if the unit is mounted on the ceiling, the keystoning is preset so the projector is even with the top of the image.)

What if the projector is off-center?
Recently there have been exciting advances in the world of image correction. While vertical keystone correction has been a reality for some time, horizontal keystone correction is new. Horizontal keystone correction is necessary when the projector must sit slightly off center — for example, in a ceiling-mount situation in which there is an obstruction. This exciting new technology means installation projectors can be positioned 10 degrees to the right or left and still center the image on the screen. Tabletop projectors can be positioned with greater flexibility as well.

Generally speaking, it is quite possible to get a perfect image if a projector has solid keystone correction.

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