Projector Keystone Correction
A must have feature
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.
So what does this all mean?
A projector lens is made to shoot above the projector, while the projector lies flat, because if you had to tilt the projector to force it to shoot up. It would cause the image on the screen to lose its rectangular shape and become trapezoidal in nature. The resulting trapezoidal image is a keystone problem and can be corrected using the methods below.
Fixed keystone correction is when a projector lens shoots the image higher than the lens so the projector itself doesn’t block the viewer’s line of sight, as shown below.
If projector lenses were not made with “fixed keystone correction“, the projector’s image would land both on and off of the screen as well would be directly in front of the viewer, as shown below.
When a projector is not set perfectly perpendicular to the screen, it will cause a distortion of the image. For example, if the projector is placed on a coffee table in front of you but the table is so lww that, even though the projector shoots up, it still doesn’t shoot high enough on the screethe projector and needs to be tilted up further upward - see below..
The resulting image below shows the trapezoidal image that is caused by the projector being tilted upward as shown in the above example.
The resulting trapezoidal image is referred to as “keystoning,” or a keystone problem. Fixed keystone correction is the ability of most projectors to shoot upwards, which makes tilting the projector upward less necessary. If fixed keystone correction is listed on a specification sheet, it will give the angle degree that the image shoots upward.
Digital keystone correction is a function that most, if not all, projectors have that enables the user to digitally correct keystone problems when the end user is forced to tilt the projector further than the fixed keystone correction allows for.
Digital keystone correction is a function of the projector’s scaler. The projector resizes the image so that the image will appear to become square again as shown below.
This image manipulation is called vertical keystone correction. Vertical keystone correction also works if the reverse has happened and you need to tilt your projector downward instead of upward as would be the case if the projector is mounted from the ceiling upside down.
While vertical keystone correction is a common feature found on most projectors, horizontal keystone correction is much less common.
Horizontal keystone correction is needed when the front face of the projector cannot be placed parallel to the screen surface as seen below. 
Horizontal keystone correction will correct the image the image as shown below.
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.
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 mount.
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.