Whether you are an on-the-go presenter working in unknown environments regularly or a home owner that needs a little more flexibility when it comes to designing your home theater, keystone correction is a must-have feature for screen projectors.

Keystoning Defined

Keystoning is caused by the projected image not being perpendicular to the screen, causing the sides of the image to distort and become different widths. Typically this is a result of the distance from the lens of the projector to the top of the image is much greater than the distance to the bottom of the image. Previously, the only way to correct the problem was to either move the projector so that the lens was closer to the center of the projected image or move the screen to achieve the same effect.

"Keystoning" occurs when a projector is aligned non-perpendicularly to a screen, or when the projection screen has an angled surface. The image that results from one of these misalignments will look trapezoidal rather than square. There are two different types of keystones: Horizontal and Vertical. Depending on where you place your projector you may only get one or possibly both keystones.

Vertical Keystones occur when a projector is mounted or placed either above or below the center of a projection screen; the projector will create an image that is wider on the top than on the bottom, or vice versa. Horizontal keystones occur when a projector is placed at an angle either to the right or left of the projection screen.

Keystone Correction: Digital and Manual

Many of today's screen projectors, when they receive data from the data source, digitally manipulate (or scale) the image to compensate for keystoning. This allows the projector to digitally correct the problem before the image even reaches the projector lens. The resulting image is always square and enables presenters and users to have increased flexibility in where they set up their presentation or home theater projector. Manual keystone correction requires a physical movement of the projector lens so that the image is projected at a higher or lower angle.

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.

Extent of Correction

Although there is not a standard degree of keystone correction, most screen projectors do offer anywhere between 15 and 30 vertical degrees of correction. The emerging trend in this area is the development of horizontal keystone correction. Horizontal keystone correction allows the projector to be slightly off-center of the desired projection area. This is extremely beneficial for on-the-go presenters who need increased flexibility and for home theater enthusiasts who have to work around fans or beams with a projector ceiling mounted.

Additional Keystone Correction Considerations

Although keystone correction is a great feature for those tough situations, it is still a better choice to make sure your home theater or presentation projector is perpendicular to the projection area to obtain ideal picture quality. Because keystone correction occurs within the projector internally, some image quality is compensated to scale the image so that it is square. Depending on the degree of the keystoning, this loss of quality may vary between subtle and severe at larger angles. This may not be as noticeable with business presentations that rely on PowerPoint and similar applications; it will be noticeable in home theater situations where image and video quality are extremely important. For these situations, it is recommended to look for a screen projector that incorporates lens shift which achieves similar results without the loss of image quality.