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Epson Projector range - What is the purpose of the Keystone correction setting?


  • Epson Projector range
  • Keystone correction


Keystone correction is a feature found in front projectors designed to compensate for mounting situations when the centreline of the projector's lens is not perpendicular to the screen to allow greater mounting flexibility. A small adjustment may also need to be made if a projector has been tilted using its front adjustable foot, thus adjusting the angle of the projector.

Many Epson projectors have a manual "Keystone" function, which allows the user to correct (vertical) keystone distortion in projected images. Some Epson projectors have the option to manually correct both horizontal and vertical keystone distortion (H/V-Keystone), as well as a "Quick Corner" function, and "Auto V-Keystone" (an automatic keystone correction function).

For this example we use a native resolution of SVGA (800 x 600). These are the pixel dimensions of the image area that the projector projects. The EB-S6 is an example of a SVGA projector. For some projectors, the resolution can be higher (depending on the source) and the projector's resolution e.g. XGA (1024 x 768), 720p (1280 x 720 or 1280 x 768), WXGA (1280 x 800), SXGA (1280 x 1024), or 1080p (1920 x 1080). 

When the projector is square with the screen both horizontally and vertically then the projected image will use all of the available projected area and be at a ratio of 1:1.

See the image below:

The dimensions of the image are 800 x 600 pixels.

If the projector is not square with the screen then perspective distortion of the screen will result. See below:

The image is still 800 x 600 pixels.

What the keystone does is to digitally resize either the top or the bottom of the projected image. This is not corrected in the optics, it is done by re-mapping the pixels. See below:

The projected image is now 800 at the top by 730 at the bottom by 600 pixels high. The background or maximum projected area is still 800 x 600.

The remapping of the 800 pixels to 730 pixels will cause loss of data. This will be most noticeable in the hard boundaries such as the text and less noticeable in images. There can be problems in the text because the pixels projected on the screen are no longer stacked vertically but are now at an angle due to the perspective distortion caused by the projector not being square with the screen.

When projected on the screen the image will appear as below:

The size of the background is still 800 x 600.

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