Keystone Correction

Keystone is the effect of not pointing a projector straight at the screen, resulting in opposite edges of the picture sloping towards each other rather than being parallel. Frequently this is unavoidable - the projector needs to be "pointed up" or "pointed down" at the screen. Many early projectors had no means of correcting this. Most modern ones do, and the correction is carried out either by optical adjustment of the lenses, or by digitally altering the image to correct for the problem. Optical correction is superior in that it does not loose any screen resolution. Digital correction, especially if operating at very steep angles, is inclined to generate moiré patterns in the image. Ideally, an AV installation should be designed to use as little keystone correction as possible. It should be considered as a last option - not a "fix all" solution to projector positioning. Horizontal keystone correction is much less common than Vertical. Where both are used together at extreme angles, there may be severe loss of resolution, and patterning. In any case, the use of digital keystone correction reduces picture brightness.

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