What Is Keystone Correction and How Do I Use It

What is "keystoning," how do you avoid it and what can you do when it can't be avoided? Read on to find out.

Keystoning is what occurs when a projector is not positioned perfectly squared and centered as compared to the display screen. The keystone effect (also called the tombstone effect) distorts the projected image from a rectangular shape with 90 degree angles at the corners, to a trapezoidal shape (much like the shape of an architectural keystone; hence the name).

Sometimes it is not enough to merely move the projector's position, as keystoning can also occur with an uneven (or not-quite level) surface on which the projector is resting.

The keystone effect can also occur when the projector is properly positioned, but the screen is not. In either case, the end result is unpleasant (and often disorienting) to the viewers. Typically, the automatic focus feature is most useful only when the projector is completely centered and squared off to the screen, and with keystoning, the full frame can seldom be in focus.

Unfortunately, it is not always possible to simply square off, tilt or center a projector in order to remove the keystone effect, and that's where a keystone correction feature can be helpful.

The types of keystone correction are: manual vs. digital; and vertical vs. horizontal.

Manual keystone correction would involve either moving the projector, or manually adjusting the projector up or down, via an adjustable leg underneath the unit. Some units feature a manually adjustable lens that can offset keystoning, and this can appear in either vertical only format, or a combination of vertical and horizontal keystone correction effects.

Digital keystone correction occurs virtually automatically, allowing any image to automatically be projected with squared corners, regardless of the angle of the actual projector. This is a fairly new feature, and is not available on all projectors.

Vertical keystone correction is useful for removing the trapezoidal effect that occurs when a projector is either tilted downward or upward. Horizontal keystone correction is useful when you are projector from a non-centered point (either to the left or right of the center of the screen).

The degree of keystone correction varies from projector to projector. For example, the range of vertical keystone correction can span from just 12 degrees to a whopping 35 degrees. You can easily check the specs on your individual projector or rental projector to see what kind of keystone correction range the projector features.

The downside of using keystone correction is that it rarely produces as crisp an image as when the projector is perfectly centered and squared. Whenever possible, position your projector so that it is displaying a squared image, or use your projector's lens shift feature. You'll feel more confident and your audience will thank you.