Keystone Correction Explained

What is keystone correction and why is keystoning a problem?

If you are in the market for a new projector, you will no doubt have spent time studying the technical specifications of the projectors you are interested in, and seeing how these projector attributes compare. These technical specifications and how they differ will affect the way in which your whole audiovisual set up performs, and so it is very important to understand each projector specification to make sure you get the best value for money.

When comparing LCD or DLP projectors, you will probably have seen projector specification sheets refer to a projector’s "keystone correction" ability. You may also have noticed that some projectors offer vertical keystone correction, while others may offer both vertical keystone correction and horizontal keystone correction. Those with horizontal keystone correction facilities are often more expensive.

You may be asking yourself:

"Do I need horizontal keystone correction as well as vertical keystone correction?"

This Bamboo AV advice article will help you to understand what keystone correction is and why “keystoning” needs to be corrected in the first place. This will help you to make sure you get a projector with the keystone correction functions necessary for your presentation application to deliver the best viewing experience possible to your audience.

What is keystoning?

Presenters, especially road warriors or others who have to frequently give presentations in unfamiliar surroundings, will often find themselves in situations where the projector they are using is not aligned perpendicularly to the projection screen, or where the projection screen has a surface which is angled. Even more commonly, users will need to position their projector on a ceiling or table which again means that the alignment of the projector will not be perpendicular. As a result a distracting problem known as “the keystone effect” occurs, which causes a distorted trapezium shaped image to be projected rather than the desired square or rectangular shape.

When the screen and projector are not perpendicular to each other, keystone correction is needed

The top image below shows the trapezoidal image that is caused when the front face of the projector is not placed parallel to the screen surface. This problem requires horizontal keystone correction.

The bottom image below shows the trapezoidal image that is caused by the projector being tilted upward or downwards at the projection screen. This problem requires vertical keystone correction.

Trapezoidal images requiring horizontal keystone correction and vertical keystone correction

Keystone correction, either manual or digital, is a technique projectors use to alter and reconstruct the outputted image to alter the shape of the image, and project it as a rectangular image. This is an option far preferable to physically moving the projector to correct any image distortion. Keystone correction is a digital technology essential to anyone using projectors to deliver a clear image, without distracting distortions. 

How does manual keystone correction work?

Manual keystone correction refers to the physical adjustment of the lens inside the projector to alter the angle that the image is projected at, without having to physically tilt the projector itself. However, the number of degrees that the lens can be adjusted by is typically limited to fifteen degrees, and the projector must be positioned quite a distance away from the screen to have a visible effect. Furthermore, manual keystone correction can only adjust vertical angles and cannot address the problem of horizontal keystoning.

What is digital keystone correction?

Digital keystone correction, on the other hand, means that the projector will use internal image scalers to digitally correct the image signal before it even reaches the lens, so the user can project at greater angles than manual keystone correction can provide. Since the user can be sure that the projected image will be a proper rectangular shape, he or she then has greater flexibility in deciding where to position the projector.

The varying degrees of keystone correction

The extent and type of the keystone correction available varies depending on the projector you plan to use. While fifteen degrees of keystone correction is a standard common amongst most projectors, some projectors will offer up to thirty-five degrees. However, it is important to note that this figure will usually only refer to vertical angles of keystone correction. Horizontal keystone correction is still a rarity among projectors, although Sony did recently demonstrate a number of projectors with these capabilities.

Keystone correction and lens shift

In truth it is preferable to use projectors with the lens shift feature instead of digital keystone correction as lens shift will generate better quality images. This is because lens shifting does not tamper with the image, and instead alters the position of the lens inside the projector. Digital keystone correction on the other hand compresses the image and recreates pixels. The scalers used in this process cause a decrease in the image quality and occasionally visible artifacts appear on the side of the projected image. This is a problem particularly noticeable when displaying fast moving videos.

The best possible scenario to maximize image quality is to not use either lens shifts or digital keystone correction, and instead line the projector up perpendicularly to the projection screen. However, this is not always a viable option and as such digital keystone correction offers the user much greater flexibility when setting up their projector. This is particularly useful for portable projector users needing to set up a presentation quickly, or home theater owners who need to work around home furniture.

We hope that this Bamboo AV advice article has been useful to you!

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