You can’t always put your projector in the perfect spot, or maybe you can, but the perfect spot doesn’t create a perfect image. Instead of a rectangular and uniform image, you see your image as a bizarre trapezoid, distorting your projection. These trapezoids are shaped like the central stone in a wedge stone arch, called keystones. Those keystones are actually the “key” to holding the arch together, but with projectors, they can make a presentation fall apart.

As long as you place your projector at an angle to the projection screen, you’ll always end up with a keystone. There are two different types of keystones: Horizontal and Vertical. Depending on where you place your projector you may only get one or possibly both keystones. Most projectors today can easily fix these skewed images with a feature called keystone correction. It is important to know how each of these keystone images are created so as to place your projector in an optimal position.

Vertical Keystones

When a projector is mounted or placed either above or below the center of a projection screen, the projector will create an image that is wider on the top than on the bottom, or vice versa. This is actually the most common keystone encountered because of the popularity of ceiling mounting a projector. If we were to mount a projector on the ceiling without adjusting the keystone and project a regular 4:3 image, it would look like this:

As you can see, the picture is skewed. If it were a spreadsheet or PowerPoint, it would most likely be somewhat illegible. By adjusting the vertical keystone, you can easily return the image to its proper look. After a 15 degree correction, the above image returns to its proper perspective:

Horizontal Keystones

Less common than vertical keystones are horizontal keystones. Horizontal keystones occur when a projector is placed at an angle either to the right or left of the projection screen. Since most projectors are mounted directly in front of a screen, this is often a lesser used feature. Still, you can’t always predict your projection environments, and may need to adjust the horizontal keystone. An example of a horizontal keystone would be to take the projected picture above and project it from the left of a projection screen to get this resulting image:

Again, with a simple adjustment of 15 degrees, the image would return to its proper perspective.

Horizontal and Vertical Keystones

If the projector is placed in an awkward place, both a horizontal and vertical keystone may occur at the same time. The resulting image is skewed to a terrible picture:

Still, as with before, we can easily adjust the keystone corrections for both the horizontal keystone and vertical keystone and return the image to its proper perspective.

Conclusion

As demonstrated above, it is very important to choose a projector that has keystone correction functionality. While not all projectors have horizontal keystone correction, most decent ones do at least have vertical keystone correction. When reading the specifications for a projector, be sure to weigh its features into your final decision on which one to choose.

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