Projector Keystone Correction and Lens Shift

It can be quite a challenge to set up your projector screen and video projector. Ideally, you should have the right placement, throw distance, and center so that when you turn your projector on the projection is placed squarely on the screen or at least at the center of the wall if you’re not using a screen. With that said, what’s the deal with Projector Keystone Correction and Lens Shift? Sometimes, you have no choice but to use digital means to fix imperfections in placement.

Keystone happens whenever your projection looks less rectangular or square and more trapezoidal in appearance because you have no choice but to project at a strange angle. Keystone adjustment or correction allows you to correct this distortion digitally or electronically.

Setting Up a Video Projector with Lens Shift and Keystone Correction

When you set up a video projector for home theater or business presentation purposes, you’ll usually have zoom, shift, and focus controls included to help project the right image on the exact right place of your projector screen. It also helps make sure that the projection is clear, sharp, sized correctly, and popping in all the right ways. With that said, the best way to adjust your projector relative to the projection screen or vice-versa is physical.

For example, you can move the angle of the ceiling mount or just the adjustment feet of the projector or projection screen to make sure the projection ends up where it’s supposed to be. Failing that due to circumstances you can’t control, like the shape of the room, to the instability of the floor or table, or the weird angle you have to project the image through (resulting in a trapezoidal shape), you can use keystone correction and lens shift to correctly display the image.

06 Projector Keystone Correction and Lens Shift 02

Keystone Correction versus Lens Shift

What’s the difference between keystone correction and lens shift anyway? Keystone correction distorts the image in order to make up for keystoning or the keystone effect that makes your projection look as trapezoidal as a keystone (hence the name). Meanwhile, lens shift enables you to physically or mechanically move the lens side-to-side or up-and-down as well as diagonally without moving the projector itself. To wit:

  • More about Keystone Correction: Keystone correction or digital keystone correction digitally alter the image or projection in order to make up for keystoning or trapezoidal distortion due to the angular placement of the screen or the wall. Ideally, you should have the projector and the wall or screen parallel to each other to straightforwardly display the square or rectangular image (depending on the aspect ratio of what you’re watching).

However, because it’s a digital alteration similar to digital zoom, this often results in a downgrade in resolution. If you don’t want your 4K to look 1080p or even 480p due to all the keystone correction you’re doing to it, you better find a setup without slanted walls or projecting your projection at an angle.

06 Keystone Correction

  • More about Lens Shift: Most projectors have a mechanical feature that allows you to shift the lens of your projector in any which way you want in case the projection is a little askew from the screen. You can even shrink the projection through zooming in or out. Usually, there’s some sort of dial or knob present that enables you to manipulate the lens in any which way you need it to move. 

There are even a number of expensive projectors that enable you to lens shift through remote control so that you can adjust the projection from afar without getting up and fiddling with any lens shift controls on the projector itself. In short, the lens shift allows you to move the projection without moving the projector and especially if you can’t move your projector for whatever reason.

06 Lens Shift

Both keystone correction and lens shift enable you to make changes to the location and shape of the image or projection without moving your projector or altering the angle of your screen to better accommodate projector placement. 

Lens Shift and Keystone Correction Downsides and Warnings

There are downsides you should watch out for when it comes to the lens shift and keystone correction. You should keep in mind the detriments to using these features because keeping your projector squarely on the same level as the wall or screen you’re projecting on without having to deal with funny angles is always your best bet towards an optimum viewing experience.


  • Keystone Correction Limitations: Even though digital keystone correction allows for both horizontal and vertical image manipulation, not all projectors include both choices. Most only opt for horizontal manipulation while a few others feature vertical and some feature both. Also, because keystone correction is a digital process, scaling and compression are used to manipulate projection shape. This naturally results in various consequences such as decreased resolution, image distortion, and artifacts or flaws.


  • Vertical and Horizontal Lens Shift: Lens shift is also known as vertical shift and horizontal shift. Like with keystone correction, there are some projectors that only have the vertical or horizontal lens shift feature. The more expensive ones will feature both. Vertical shift refers to the ability to shift the lens up or down, enabling you to put the askew projection higher or lower. Horizontal shift refers to moving the image left and right when it’s askew on the side of the screen.


  • Projecting at an Angle: Keystone correction corrects keystoning by changing the image itself from the source to allow an off-center projector to project a video, app, user interface, or slideshow presentation at an angle. When a projector is tilted upwards or downwards, it ends up showing a pyramidal shape without the top part. When a projector is tilted to the side or if the wall is angled sideward, this then results in a trapezoidal image that slightly tapers to the left or to the right.


  • Various Resolution Issues: Keystone correction, lens shift, or digital zoom can typically result in lower resolution. Your 1080p HD image can turn into 720p or lower. Ditto with 4K being reduced to 1080p. As for lens shifting, it’s more ideal to center the projector than to shift the lens off-center just to place the projection square on the screen. Also, while both lens shift and keystone correction can correct faulty positioning and projections, they serve different purposes.


  • The Eye of the Beholder: The more levels of keystone correction you use on a trapezoidal image the more obvious the degradation is. However, if the adjustments are slight then it’s not really a big deal and only through careful observation can the audience notice the difference. If it’s just one click of keystone correction, the pixel loss is practically nonexistent or unobservable. However, with a few more clicks it becomes a subjective issue of how much degradation is too much. At maximum clicks the resolution loss is most obvious.


  • Using Lens Shift and Keystone Correction Together: Using lens shift with keystone correction together—even though lens shift isn’t as “digital” a feature as keystone correction—can lead to more lowered quality or even additional distortion. Lens zoom with keystone correction is especially damaging to even 4K projections. It’s like how adding more filters to an image through Photoshop or Instagram filter can lead to more artifacts or flaws on the image as well as pixilation and blurring. It can even go to the point of making the image unrecognizable.

The Lens Shift and When It's Most Applicable to Use

The Lens Shift and When It’s Most Applicable to Use 

Projection degradation is less of an issue when using lens shift since it’s instead about shifting the lens of the projector instead of digitally distorting the image it’s projecting. It’s like the difference between a lens zoom and a digital zoom—lenses retain more quality from their zoom while a digital zoom works with the image the lens has captured by zooming into part of a digital picture.


  • When to Use Lens Shift 101: Lens shift vertically or horizontally should be used if the projector is properly aligned with its wall or screen at a perpendicular angle. It’s not particularly noticeable if the discrepancy is small. However, don’t use it when the projector is at an odd angle with the screen, resulting in an uneven image that’s narrow on the top, bottom, or one its sides. You should use keystone correction to fix that trapezoidal flaw to widen that narrow gap until you get a square or rectangular result.


  • The Variable Lens Shift Feature and Price Concerns: Some projectors have what’s known as the variable lens shift feature. It’s typical of DLP projectors priced at $3,000 or under and not so much lower-cost LCD ones. It’s not a common feature for entry-level projectors though. You actually need to step up the plate when it comes to your budget in order to get a decent lens shift that features both vertical and horizontal lens shift, allowing you to freely manipulate the projection image in case it’s askew by both levels. At $2,999 you can get a DLP projector with this feature available.


  • Areas of Compromise: Sometimes a projector will feature full variable lens shift for up, down, left, and right movements in exchange for a lower quality zoom lens. You might end up with a projector that only has a 1.2:1 zoom ratio instead of the preferable 1.3:1 in exchange for the variable lens shift. In turn, if you prioritize zooms more than lens shift because your home theater has a fully adjustable screen or table by which your projector is placed on, you can opt for a 1.3:1 zoom ratio projector that only has vertical lens shift.


  • Placement Distance: Another consideration when it comes to gauging lens shift quality is placement distance. For example, a projector might need 13 to 15 feet of space or throw distance in order to project a 100-inch diagonal screen. Lens shift might be necessary when placing a projector on a shelf at the back of the room instead of a ceiling mount. Since the shelf isn’t made to accommodate projector throw distances, lens shift adjustment might be called for.


  • Proper Projector Mounting 101: When mounting a projector, you’ll need the center of its lens to be slightly above the top of the projection screen surface. If it’s sitting on top of a table, the center of the lens, you need the center of the lens to be a few inches below the screen’s bottom instead. You’ll need a ceiling mount installed if you wish the projector was high up enough to invert it. You can alternatively just put it up a mount or a tripod you can adjust to save you the trouble of vertical or horizontal lens shift.

The Debate on Keystone Correction and Quality Degradation

The Debate on Keystone Correction and Quality Degradation

Does keystone correction result in quality degradation? Probably, especially if you have to do 5 clicks to correct a distortion, only to end up with a different type of distortion on the resulting projection. Since correcting keystoning is a digital process instead of physical manipulation, you only have so many pixels to work with. Even full 4K resolution at  3840 x 2160 pixels or 4096 x 2160 pixels will lose enough pixels to drop down to 1080p when enough keystone correction is used.

  • The Cause of Keystoning: Projecting your projector at an angled wall or having its lens angled at a straight wall in order for your audience to see the projection better because you don’t have an aisle for the device’s lamp to shine through are among the reasons for keystoning. It’s either the surface is angled or the projector is angled. Keystone correction is necessary when you have no choice but to use it in order to get a non-trapezoidal projection. Maybe you can’t move the projector or alter the wall. Maybe the screen is too small the angled wall is bigger.


  • Progressively Shrinks The Number of Pixels: Keystone correction progressively shrinks the number of pixels the more clicks you apply on it. This is the same deal when you do other types of digital adjustment ranging from stretching to a different aspect ratio to make a 4:3 video fit a 16:9 screen or doing digital zoom ins and lens shift. A mathematical adjustment must be done per correction or feature applied, resulting in rounding errors and jaggedness of lines. The pixels will shrink vertically or horizontally to make up for the distortion or adjustment, like a funhouse mirror distortion being corrected to the original image.


  • From 4K to 1080p to 720p and Below: The projection won’t necessarily go down from 4K to 1080p when keystone correction is applied but it might progressively shrink in resolution like a scaled down image in Photoshop. In order for the change to be drastic you need at least five clicks of keystone correction. However, even the first click will have you lose details because you’re distorting the original image in order to fix another distortion. Pixel loss will naturally happen here as with stretching an image to fit a new aspect ratio. Your 4K resolution can incrementally drop down to 1080p or lower, with straight lines becoming jagged and pixilated.


  • Allegedly Half of Visible Resolution: Digital keystone correction can kill off almost half or fifty percent of the projection’s visible resolution, especially if you do five clicks. First click is only slight. You might not even see the difference if you’re a casual video viewer who’s even good to go with 480p or 360p resolution on YouTube. Hey, as long as it’s viewable then there shouldn’t be any problem, right? However, for the most part it’s 4K to 1080p, 1080p to 720p, or even 720p to 480p in terms of resolution degradation.


  • Viewing Distance Considerations: Like with lens shift, you also have to deal with throw or viewing distance. When you’re looking at a keystone-corrected projection you can see all the flaws, blown-up pixels, image distortions, and artifacts, If you’re far enough away, it stops being so obvious and it might even do a good enough job to be passable. Keystone correction doesn’t result in something unwatchable unless the keystoning is so steep and extreme that you have to do five clicks and see severe image degradation as a result. Quality is subjective but raw numbers on lost pixels are not.


When All Is Said and Done

So is keystone correction and lens shift worth it or not? As much as possible, address the alignment issues when installing the projector before considering fixing the keystone effect with keystone correction or moving the lens of the projector instead of the projector or projection screen itself. The projector can only work with so many pixels. If you have to do lens shift, you want about 20 percent distance flexibility on the screen to minimize image degradation.

Even though digital keystone correction and lens shift are useful, minimize their use. Use them only as a last resort instead of a first one. Your first resort is finding a way to project without funny angles or uneven floor levels. Have your projector dead center to the screen in a way that’s viewable to all your audiences. You can even purchase a short-throw projector if you’re concerned that people will block the view or light of the projector itself.  Sometimes, you have no choice but use these digital correction tools despite resolution and quality loss. It cannot be helped.

Image Credit:

James Core

I love my projector system and I am here to help you find the right projector for your needs.

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