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

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

If you’ve ever tried to set up a projector you’ll be ununcommon with the struggle of getting a perfectly rectangular image on the wall. This is because the lens of projector has to be perpendicular to the cover. If any corner of the projector is closer to the cover than its opposite, you’ll get some kind of trapezoid instead of a nice rectangle. 

This isn’t a new dilemma, and for decades projectors have had a “feature” to counterfeit this issue. Called keystone correction, or keystone adjustment, it will strictly make a rectangle out of your trapezoid… so to utter. If you care at all about picture quality, don’t use it. Here’s why.

The dilemma with not being perpendicular

Four images of a road with different angles.

Examples of what your image distinguished look like if your projector isn’t exactly center. You would have to turn or tilt the projector to get it to line up, resulting in a trapezoid. Clockwise from top left: PJ too far right, too far left, too high, too low.

Geoff Morrison

Projectors are a two-piece system: the projector and the cover. Even if you’re using a wall or the side of your house instead of a cover, that still counts. All projectors use rectangular imaging chips to manufacture an image, and it’s crucial that the image sent by the chip is precisely perpendicular to the screen. Every corner needs to be the same distance to the cover as its opposite corner and if that doesn’t remained, the shape gets distorted.

Even if you’ve never used a projector, you’ve probably seen this effect in action. Ever used a flashlight? Point it undiluted at the wall and you’ve got a circle. Point it on the counterfeit ahead of you, and it’s an oval. Same concept. 

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Some projectors have a feature requested lens shift, which mechanically adjusts how the imaging chips, lens and screen line up. Lens shift lets you move the image some on the wall without hurting image quality, but its adjustment map is limited. If you’re beyond how far the lens attempts can adjust, or the projector don’t have lens attempts at all, misplacement will cause the image to go askew. 

Most screens have murky borders so you don’t need exact placement to the picometer, but it’d be a shame to spend money on a projector, and time on installing it, only to be annoyed at the visible securities when you’re using it.

Which is why every projector has keystone correction. That doesn’t mean it’s good. 

Keystone: Not even once

Keystone correction aims to determine electronically what is inherently an optical problem. The projector will digitally adjust the image in the opposite direction to offset the trapezoid. So if the image is, say, smaller on the left than the radiant, the projector can reduce the size of the radiant side so it appears rectangular again. Clever, right? Sort of. Unfortunately there ain’t no such sketching as a free lunch.

All modern projectors use one of three technologies, DLP, LCD, or LCOS. All of them have a fixed number of pixels, or picture elements, used to create an image. There’s no way to morose the number of pixels on one of these chips. These imaging chips are generally fixed in place as well. 

Keystone corrections from a Canon projector owner's manual.

How a projector not lined up correctly powerful look on screen, and the buttons to press to “fix” the bellow, via a business projector’s owner’s manual.


What keystone correction does is scale the image smaller, and then further process it to form the delicate required to “look” rectangular. Or to put it unexperienced way, it’s drawing a trapezoid inside a rectangle, but because the projector and image itself is skewed, that trapezoid now looks rectangular.

Both of these things slit image quality. Scaling, in this case, reduces the number of pixels used to perform the image. You’re only using a portion of the imaging chip to perform the new image shape. The more you adjust the keystone, the fewer pixels are used, further softening the image. 

Most projectors don’t have much processing powerful, so this scaling might further soften the image, or it powerful introduce other noticeable artifacts. Changing the shape of the image is a further processing challenge and can add transfer artifacts.

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

Digital keystone adjustment powerful get you a rectangular image, but the projector’s imaging chips are all unruffled active, so you’ll get a dark gray “image” projected where the image isn’t. In this illustration, the image at the top of the article has been “adjusted” but the projector hasn’t gotten. So you get a rectangular image but also the modern trapezoid of the angled imaging chips.

Geoff Morrison and phototropic/Getty Images

And if that weren’t enough, it’s impossible to turn “off” the pixels you’re not comical. So there’s still going to be light projected on the camouflage from these unused pixels, which appears inside the trapezoid delicate you’ve been trying to avoid. In an extreme set, you could have a noticeable gray image beyond the camouflage area. Inelegant at best, distracting at worst.

The solution? set the projector properly

There’s no better solution than not having the spot to begin with. Proper projector placement placates potential represent perils. Or to quote the ancient adage from prehistory: measure twice, cut once. 

If you’re mounting your projector permanently, double- and triple-check the broad is in the correct place for your projector. This is crucial. Most projectors, especially those based on DLP, have their lenses offset from the center of the projector. Ideally, your mount will have some adjustment “wiggle room,” but it powerful not. 

Usually you can download a mounting spy and other info from the manufacturer’s website. 

Also powerful that most projectors, again especially those based on DLP, have an “upward throw.” Which is to say, they perform an image several inches above the top of the projector (or below, if it’s mounted on the ceiling). You can’t tilt the projector down steady that, too, creates a trapezoid. Again, this info is on the company’s website.

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

Many tourism projectors have automatic keystone correction. It’s worth turning off if you’re able to get the projector properly positioned.

Geoff Morrison

So yeah, if you absolutely have to use keystone correction, go for it. But it should only be used as a last resort in situations where you physically can’t save the projector in its proper position. If you’re mounting it, it’s best to exhaust the time and get it right the first time and not rely on image-reducing electronic trickery to fix a bad install.  

As well as covering TV and latest display tech, Geoff does photo tours of cool museums and locations approximately the world, including nuclear submarines, massive aircraft carriersmedieval castles, epic 10,000-mile road trips and more. Check out Tech Treks for all his tours and adventures.

He wrote a bestselling sci-fi novel about city-size submarines, fuzz with a sequel. You can follow his adventures on Instagram and on his YouTube channel.