Keystone correction and vertical shift might have an effect on picture quality. Just like in the case of digital zoom wherein the resulting picture ends up a blurred zoomed-in version of what the lens caught from a distance without capturing the quality of a lens zoom, a projector’s keystone correction might affect the quality of what you’re watching.
So does keystone correction reduce resolution? It does. It should, therefore, be used as a last resort if you aren’t able to adjust the keystoning effect by yourself and the physical placement of your projector.
The Debate on Keystone Correction and Quality Degradation
So keystone correction or keystoning can reduce quality. Let’s discuss why that’s so. It’s because it’s a digital process rather than one involving physical manipulation of the projector and its lenses. Naturally, the more you use keystoning to allow you to project your movie in an angled or uneven surface, the more the quality of what you’re watching suffers, with the resolution shrinking and whatnot.
- What Is Keystoning? The keystone effect on projectors is image or projection distortion caused by projecting it on an angled surface. The image dimensions are altered or distorted from looking like a square to a trapezoid, which is also shaped like an architectural keystone. Keystone correction or keystoning corrects that distortion, allowing your skewed output image to become square again without moving your projector. This allows you to do things like watch videos on an angled or uneven surface, turning a trapezoid to a square regardless.
- Progressively Shrinks The Number of Pixels: Whenever you do any sort of digital adjustment—zoom in, stretch to a different aspect ratio than the native aspect ratio, keystone correction, lens shift, or vertical shift—the mathematical adjustment will progressively shrink the number of pixels horizontally or vertically in order to work with a given resolution of video or user interface. It’s a mathematical process with rounding errors involved. You could end up with straight lines that look jagged and an image that loses sharpness or at least a lower resolution.
- From 4K to 1080p to 720p and Below: The lower the resolution of an image, the more you lose details. The higher the resolution you can work with, like in the case of 4K, the lower the pixel loss will be because you’re working with loads of pixels to begin with. However, like in the case of cropping or stretching a picture, you don’t end up with more pixels but less every time you make use of a digital process adjustment. Also, higher resolutions can’t account for jaggedness of straight lines as though you’re looking at PlayStation 1 polygons.
- Allegedly 50% of Visible Resolution: Some projector enthusiasts argue that digital keystone correction tends to kill off 50% of the visible resolution of the image. You can go from 4K to 1080p HD by keystoning or 1080p to 720p quality. It just gets worse and worse as you go down the list of resolutions available, to the point that if you were to watch a 144p video, keystoning makes it a virtually unwatchable pixilated mess.
- Pixilation in Exchange for Convenience: This resolution drop is in exchange for its one-click or one-button convenience. It adjusts and corrects keystoning almost automatically, so expect some artifacting to happen as a result. This is also a trade-off that you can get when using too much lens shift or vertical shift. You aren’t able to shoot through the most accurate part of the projector lens, which is the center. Blurring and pixilation everywhere! It’s as if the default flat projector screen facing a projector squarely is the best option.
- Using Lens Shift and Keystone Correction Together: Using two or more digital correction techniques at the same time will naturally lead to even more lowered quality compared to using only one digital process such as only lens shift or only keystoning. It’s like how the more filters you put on your Photoshop or Instagram image the more the image itself becomes distorted by layers and layers of filters.
- Viewing Distance Considerations: Up close you’ll see all the imperfections. However, there are times when your setup requires you to use substantial vertical and horizontal shift on top of keystone correction. If the audience is far away enough or is at a decent viewing distance, they shouldn’t be able to detect any image problems unless they go up close to the projection itself and examine the pixels one by one. You won’t detect any image problems if you have resolution to spare and you’re at a healthy viewing or throw distance.
- How Drastic Is The Change? The results of the pixel and resolution loss from keystoning and/or lens shift (as well as other adjustments like stretching or blowing up a 720p image to fit a 1080p resolution as well as aspect ratio alterations) are only as drastic as the eye of the beholder. Those who are used to 4K or 1080p HD will have eyes that can pick apart image degradation. Most laymen couldn’t see the difference. The quality loss also depends on screen size and throw distance.
- The Eye of the Beholder: Objectively, if the keystone correction is to correct slight trapezoid distortion then there’s less pixel loss compared to complete keystone distortion (the square looks like the roof of a house, for example). However, subjectively, there are those viewers who don’t want to lose even a single pixel and believe that any keystoning from a 1080p projector automatically results in 720p degradation when it’s not necessarily the case. It also depends on your point of view on how much is too much resolution loss.
How Severe Is the Resolution Loss from Keystoning?
You won’t know how much the keystone correction process will reduce the resolution of whatever it is that your projector is projecting until you try it out for yourself. After all, for the most part, if you’re using a 4K image you have enough pixels to work with that you won’t notice the difference too much, straight lines being jagged as the image becomes more pixilated notwithstanding.
- It Can Actually Help Out: The MTF and CR of your projector might even improve slightly when using a small amount of lens shift or keystone correction. Modulation Transfer Function (MTF), also known as spatial frequency response, is a parameter used to objectively measure the sharpness of an image. CR stands for contrast ratio. As long as the adjustment is slight it can improve the image. Everything in moderation. How good the keystoning and lens shift depends on your projector’s lens quality and overall capabilities.
- Too Much Distortion Results in Too Much Pixel Loss: If the distortion is severe the pixel loss will similarly be severe when you adjust the keystoning. If you have more pixels to work with as in the case of 4K or 1080p projectors you can minimize the loss to still be on HD. You still have 1080 lines to work with and some of the horizontal lines can be scaled up or down. Steer clear of scaling if it’s possible or minimize it as much as possible. It all depends on how much resolution you’re working with.
- Image Resolution and How Much Pixels Lost: Is it necessarily half of the pixels lost when you do even a slight keystone correction? If you’re adjusting it slightly then you only lose pixels slightly. As many Adobe Photoshop owners will vouch, when adjusting a distorted picture, the quality of the corrected picture will ultimately depend on its resolution before the distortion. There’s only so much a projector can do to severe keystoning. If you’re working with 4K you can still end up with something HD and clear when push comes to shove.
- No Visible Image Degradation Usually: Unless you do a 1:1 mapping of the pixels of a keystoned or lens-shifted image, for the most part, you won’t see any visible image degradation even at the closest viewing range. Probably those who are used to 4K movie viewing or gaming and can see from their naked eye any frame drops or quality drops will take note that the keystoning has dropped the projector’s maximum 4K projection on a large screen. Those same people will try their hardest to avoid keystoning setups anyway to ensure picture fidelity.
In a Nutshell
To avoid pixilation, you should tilt the projector and screen enough to square the image by yourself or simply find a flatter surface or screen to project your projector. Use keystoning as a last resort. If you can manually adjust anything then do that to ensure resolution fidelity for your 4K or 1080p movie and videogame when push comes to shove. But to most people, the degradation and pixel loss isn’t visible in the least.
The average homeowner can do 1 to 5 clicks of keystone correction and to them the image might still be high quality in their eyes. Aside from keystone correction, there’s also a vertical shift or lens shift to consider. They all might degrade the picture since they’re digital adjustments.
It’s like how you do a digital zoom instead of a lens zoom on a given image. Even if you’re using a detailed high-definition picture quality, the zoomed-in part is still just a blown-up version of what you’re seeing. In turn, the keystoned image will lose pixels in order to adjust the correction.