Keystone Correction is easily the most widely used technique for correcting the effect of a projector’s keystone distortion. Keystone is another word for trapezoid and distortion simply means that something isn’t being shown in its real shape, proportions, or quality.
Projector distortions can be caused by many different factors but in this article, we’ll focus on the use of Keystone Correction and whether it really does reduce the resolution.
In order to test this, we set up an image to confirm that a projected image would not be square with either horizontal or vertical edges.
We then asked three volunteers to look at these images through a projection screen from two different distances: 6 feet and 10 feet away. Each volunteer rated the image quality at both distances. We used a very sharp image that we knew would be difficult to resolve.
Keystone correction was applied to the image so that each of the four corners of the projected image had been turned in by 12 degrees (one keystoning value used on many projectors). This allowed us to test whether or not Keystone Correction affected resolution.
Projector Keystone Correction & Quality Degradation
1. What Is Keystoning in Projector
Keystone is a projection alignment issue that causes the top and bottom of a projected image to be slightly wider than the left and right edges.
This problem occurs when the projector is not aligned parallel to the screen, which causes lines in an image to become noticeably distorted. Projectors with adjustable lens shift or keystone correction offer an elegant solution that keeps images looking perfectly straight on any screen type.
2. Progressively Shrinks The Number of Pixels of Projector
In the last few years, most video projectors have migrated to 1920×1080 pixel resolutions. It is a common misconception that projectors with larger pixel counts are better than their lower-resolution counterparts.
However, in many circumstances, a projector with fewer pixels can produce better image quality than one with many more pixels.
3. From 4K to 1080p to 720p and Below
As displays have evolved, they have increased in size and decreased in resolution. Originally, personal computer displays were 233 pixels across and 178 pixels high with a standard screen resolution of 640×480.
This is equivalent to 0.19 megapixels. The first high-definition television was 640×480 pixels or 0.3 megapixels. For a decade, the standard television was 720×480.
This is equivalent to 0.7 megapixels (720p). The current standard for a new television is 1080p at 1920×1080, or 2.1 megapixels. 4K is 3840×2160 which is 8.3 megapixels. This means that 4K has 4 times the resolution of 1080p, but only 1/3 more than 720p.
4. Allegedly 50% of Visible Resolution
Most projector manufacturers provide only three numbers for their pixel count: The native resolution of the projector (e.g. 1080p), the effective image size (e.g. 2,000 lumens), and the native black level in nits (e.g. 3500).
These are all very different specifications that have nothing to do with each other; they are simply marketing statements that imply incorrectly that a projector has twice as many pixels as it actually does.
The number of pixels is independent of the brightness level of a display, similar to the resolution and length of a movie playing on your screen having nothing to do with how much light your room emits at night.
5. Pixilation in Exchange for Convenience
One of the main reasons for the widespread adoption of higher resolutions is the ease of use. A larger number of pixels means that more information can be placed on a screen while retaining similar viewing sizes.
For example, a person who needs two feet of viewing distance from a display but wants to watch a football game on it could only do so using 640×480 pixels at 14-18″ viewing distances, 720p at 10″ to 12″, or 1080p at 8″ to 10″.
With 4K, the same football game could be watched from 18″away with 2.1 megapixels; 8-10″ away with 4.1 megapixels; and 12″away with 8.1 megapixels.
While the increased resolution might make it more difficult to read text, the increase in viewing distance could make it easier for someone to watch a football game while having similar eye relief.
6. Using Lens Shift and Keystone Correction Together
Many different types of interference are possible that can cause vertical keystoning problems including vertical lens shift (VLS), mechanical keystone correction, image shift due to a box or ceiling above the projector, and light intensity from a nearby source that is less than ideal.
In all of these cases, proper use of lens shift or keystone correction can correct the problem so that images remain perfectly vertically aligned on any screen size and type.
7. Viewing Distance Considerations
Projector light output is the same whether it is used to project images straight through a wall or from a few inches away. It is very important, though, that the distance between the projector and the screen be greater than the viewing distance of when it will be used.
Projectors can be designed to accept more than 1440 lumens of light for extended viewing distances, but they use very little power in doing so. If a projector will not be used at extended distances, best practices would dictate using less than 1400 lumens for both onscreen image contrast and power consumption.
8. Viewing from the Side Improves 3D Images
There are several reasons why viewing from the side improves 3D images. First, a wider field of view allows more of the screen image to be seen. Second, it is easier to be comfortable sitting further away from the screen because you can look at and then turn your head away from it without moving your head in relation to the projector.
Finally, projection with 90-degree horizontal viewing angles reduces crosstalk with a window or wall behind it making it easier to see and hear 3D content overall.
“Keystone correction is the process of adjusting an image so that the four sides are all straight and perpendicular to each other. It is most commonly used in video projectors, where the projector display is projected onto a screen or surface that isn’t perfectly rectangular. The keystone effect then occurs because of different angles between the projector and screen.”
A lot of people believe that Keystone Correction reduces the resolution on their projector but this isn’t true. There’s no need to be concerned about not getting an as high-quality picture when you’re projecting your image if you have Keystone correction enabled.