Unlike a TV, setting up a video projector is a lot more complicated. Since it is a two-piece setup, you have to line up the projector with your screen or wall at the right distance and angle to display an image of the desired size as well as make sure the image is a “perfect” rectangle (regardless of the aspect ratio – all the screen dimensions are rectangles – a square is a rectangle).
For best results, physically place the projector so that the image is the right size and geometry.
Most projectors provide focus and zoom controls to tweak the size and sharpness of the projected image. Also, projectors have adjustable feet to help adjust the angle of the projector a small amount to the screen/wall.
If you have a ceiling-mounted projector you should be able to adjust the angle of the ceiling mount. Some projectors also include auto-flip which detects if the projector is right side up or when mounted on a ceiling, upside down. Instead of going into the projector’s setting menu, the projector may automatically flip the image so that is always right-side up in relation to the viewer.
However, after you have done all that you may still find that the image may still not have the correct rectangular geometry. If the angle of the projector is not perpendicular to the screen or wall, you may see an uneven, trapezoidal image.
This may take the form of an image that is wider at the top than on the bottom (or vice versa) or slanted or uneven on one or both sides.
What Keystone Correction Is
Keystone Correction (aka Keystoning) is a setting that is used to correct the geometry of an image where the projector is not perpendicular to the screen due to the angle of the projector in relation to a screen or wall.
Depending on the model of the projector where Keystone Correction is provided, it may have one or more of the following options:
Auto: Auto keystone correction means there’s an algorithm within the projector’s processor that self-adjusts the lens angle. This works in the vertical plane – so if you have an image that is different sizes on the top and bottom edge, Auto Keystone may be able to correct it.
Manual: Manual keystone correction is a physical adjustment (usually a slider) that tilts the lens angle higher or lower. The manual adjustment works well in some situations, but it is not effective when dealing with horizontal keystoning issues.
Digital: Digital Keystone Correction allows you to digitally adjust the image before it passes through the lens using control buttons on the projector and/or remote. Depending on the brand/model of the projector, digital keystone correction may provide both vertical and horizontal keystone adjustments, or just vertical or horizontal. Check the projector’s online specifications page or user guide to make sure it has the flexibility you are looking for.
Tip: Some projectors not only provide vertical and horizontal keystone – but also the ability to fine-tune corner (4-point keystone) and mid-edge points (8-point keystone) as well.
There are even projectors that have 9 to 15-point keystone correction (aka Image Warping), such as the LG CineBeam HU915QB, HU715Q, and HU710P.
LG Keystone Correction/Image Warping Demo
Another thing to look for is the degree of Keystone Correction. This varies, but a common range for video projectors is:
- Horizontal: +/-30 degrees
- Vertical: +/-30 degrees
Tip: Some projectors may extend that out to +/- 40 degrees or as little as +/- 15 degrees. Also, the keystone range may be different for the Vertical and Horizontal settings.
Keystone Correction May Not Always Be Desirable
If you have some patience, Digital Keystone correction can provide a lot of flexibility in fixing the geometry of a displayed image, but there is a catch: Digital Keystone uses scaling. This means that the adjustments can decrease display resolution by decreasing the number of pixels used to display the image.
Also, when you are not using all of the pixels on the imaging chip to display the corrected image, you could end up with image softness.
In addition, light is still projected on the screen from unused pixels, which may result in visible gray areas displayed beyond the borders of the corrected screen image.
Using Keystone Correction may result in digital artifacts, image distortion, or decreased resolution.
Tip: Keystone Correction should be used only as a last resort – make sure you have done your best to space your projector according to the instructions outlined in the projector’s user guide. Your projector may also provide another useful tool: Lens Shift, which will be discussed in a forthcoming article.
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