What is Keystone Correction for Projectors? And Why You Should Avoid It

If you ever tried set up a projector You’ll be familiar with the struggle of getting a perfectly square picture on the wall. This is because the lens of projector must be perpendicular to the screen. If one corner of the projector is closer to the screen than the opposite one, then instead of a beautiful rectangle, you get a kind of trapezoid.

This is not a new problem, and projectors have had a “feature” to counteract this problem for decades. Called keystone correction or keystone adjustment, it will technically make a rectangle out of your trapezoid… sort of. If you care about image quality at all, don’t use it. Here’s why.

The problem of not being vertical

Four images of a street with different angles.

Examples of what your image might look like if your projector is not centered exactly. You would have to rotate or tilt the projector to align it, resulting in a trapezoid. Clockwise from top left: PJ too far right, too far left, too high, too low.

Geoff Morrison/CNET

Projectors are a two-part system: the projector and the screen. Even if you use a wall or the side of your house instead of a screen, that still counts. All projectors use rectangular image chips to create an image and it is crucial that the image sent by the chip is perfectly perpendicular to the screen. Each corner needs to be the same distance from the screen as the opposite corner, and if that doesn’t happen, the shape will be distorted.

Even if you’ve never used a projector before, you’ve probably seen this effect in action. Ever used a flashlight? Point it straight at the wall and you have a circle. Point it at the ground in front of you and it’s an oval. Same concept.

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Some projectors have one Feature called Lens Shift, which mechanically adjusts how the imaging chips, lens, and screen are aligned. Lens Shift allows you to slightly shift the image on the wall without sacrificing image quality, but the adjustment range is limited. If you’re past how far the lens shift can be adjusted, or the projector doesn’t have any lens shift at all, improper placement will cause the image to be crooked.

Most screens have black borders so you don’t need precise placement on the picometer, but it would be a shame to spend money on a projector and time to install it only to be annoyed by the visible borders when using it.

For this reason every projector has a keystone correction. That doesn’t mean it’s good.

Keystone: Not even

Keystone correction aims to solve electronically what is inherently an optical problem. The projector digitally adjusts the image in the opposite direction to compensate for keystone. So if the image is smaller on the left than on the right, for example, the projector can reduce the size of the right side so that it appears rectangular again. Smart right? Type of. Unfortunately there There is no free lunch.

All modern projectors use one of three technologies, DLP, LCD or LCOS. All have a fixed number of pixels, or picture elements, that are used to create an image. There is no way to change the number of pixels on any of these chips. These imaging chips are also usually stationary.

Keystone corrections from a Canon projector manual.Keystone corrections from a Canon projector manual.

Consult a business projector manual for what a misaligned projector might look like on screen and the keys to press to “fix” the problem.


What Keystone Correction does is scale image smaller, and then further process it to form the shape needed to “look” rectangular. In other words, a trapezoid is drawn inside a rectangle, but because the projector and the image itself are skewed, that trapezoid now looks rectangular.

Both reduce the image quality. Scaling, in this case, reduces the number of pixels used to create the image. You only use part of the imaging chip to create the new image shape. The more you adjust the keystone correction, the fewer pixels are used, making the image even softer.

Most projectors don’t have much processing power, so this scaling can further blur the image or introduce other noticeable artifacts. Changing the shape of the image is another processing challenge and can add additional artifacts.

A home theater with an illustration of what the spot of light from an incorrectly mounted projector would look like.A home theater with an illustration of what the spot of light from an incorrectly mounted projector would look like.

Digital keystone correction may give you a square image, but the projector’s imaging chips are all still active, so you’re projecting a dark gray “image” where the image isn’t there. In this illustration, the image at the top of the article has been “adjusted” but the projector has not moved. This gives you a rectangular image, but also the original trapezoid of the angled image chips.

Geoff Morrison/CNET and phototropic/Getty Images

And as if that wasn’t enough, it’s impossible to “turn off” the pixels you’re not using. So light is still projecting onto the screen from those unused pixels, appearing in the trapezoidal shape you’re trying to avoid. In an extreme situation, you could have a noticeable gray image outside of the screen area. Inelegant at best, distracting at worst.

The solution? Set up the projector correctly

There is no better solution than not having the problem in the first place. Proper projector placement mitigates potential image hazards. Or to quote the old adage from prehistory: measure twice, cut once.

If you are permanently mounting your projector, double check and triple check that the bracket is in the correct place for your projector. This is crucial. On most projectors, especially those based on DLP, the lenses are offset from the center of the projector. Ideally, your mount will have some “wiggle room” to adjust, but it might not.

You can usually download an assembly template and other information from the manufacturer’s website.

Also keep in mind that most projectors, again especially those based on DLP, have an “up throw”. That means they create an image several inches above the top of the projector (or below if it’s ceiling mounted). You cannot tilt the projector down as this will also create a keystone. This information can also be found on the company’s website.

Star-like light emanates from the front of the Smart Projector 2.Star-like light emanates from the front of the Smart Projector 2.

Many portable projectors have automatic keystone correction. It’s worth switching off if you can position the projector correctly.

Geoff Morrison/CNET

So yes, if you absolutely must use keystone correction, do it. But it should only be used as a last resort in situations where you cannot physically place the projector in the correct location. When you mount it, it’s best to take the time and get it right the first time and not rely on image-reducing electronic tricks to fix a bad install.

In addition to reporting on TV and other display technologies, Geoff also conducts photo tours cool museums and places around the worldincluding nuclear submarines, massive aircraft carriers, medieval castlesepic 10,000 mile road trips and more. Cash Tech Treks for all his tours and adventures.

He enrolled Best-selling science fiction novel about city-sized submarines, along with a Consequence. You can continue to follow his adventures Instagram and on his Youtube channel.

What is Keystone Correction for Projectors? And Why You Should Avoid It Source link What is Keystone Correction for Projectors? And Why You Should Avoid It

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