What is Keystoning?

For the best picture quality and a perfectly projected rectangular image a projector should ideally be perpendicular to the centreline of the projection screen. This is seldom achievable resulting in a trapezoidal shape projected image on the screen. The effect, keystoning, comes from the similarly shaped keystone at the top of an arch of brick or blocks. There are two types of keystoning; vertical keystoning where the projector is above or below the centreline of the screen and horizontal keystoning where the projector is to the left or right of the screen.

Recognizing the most common positioning situations, projectors commonly have a built-in fixed keystone factor allowing the projector to be placed at a shallow angle below or above the projector screen. However, this does not provide the flexibility required for all installation situations and two common methods of keystone correction employed by projector manufacturers are; Variable Optical Lens Shift and Digital Keystone Correction.

Digital Keystone Correction

Digital keystone correction involves a scaling/compression algorithm being applied to the image before it is projected. In effect, the compression algorithm squeezes the entire image down to the thinnest edge of the trapezoidal projected image. While the approach achieves the desired rectangular projected image, it does have some possibly undesirable effects including;

• apparent resolution reduced
• some dimming – more noticeable in heavily compressed areas
• possible video artifacts or distortions
• possible reduced sharpness – once again in the most heavily scaled/compressed area (affects clarity of text and graphics)

Some projectors sense the vertical inclination of the equipment with a built-in sensor and automatically scale the image to produce a rectangular image on the screen.

Variable Optical Lens Shift

Variable optical lens shift refers to the horizontal and vertical movement of the projector lens within its housing. It is a manual optical adjustment and provides a better keystoning solution. However, it is not perfect and can result in a slight bowing of the image.

Lens shift is much more commonly offered with LCD projectors than DLP projectors as adding it to the optical path of a single DLP chip projector is considerably more expensive. More expensive, three chip DLP projectors are much more likely to include lens shift.

In summary, it’s better to try and achieve the correct projector alignment rather than resort to keystone correction. If it’s not possible, ensure that the keystone correction required is as small as possible as the likelihood of degrading image quality increases with wider angles.