Overhead projectors are still one of the most common A/V aid in training environments. They need to be located within arms length of the lecturer so they can write on the light table. This usually puts the projector very close to the front wall in a training room, and there's often a need to have the blackboard available as well. The default projection surface is usually above the blackboard, which results in drastic image distortion known as "keystoning". When this is combined with a short depth of field on the projector lens, part of the image will also be out of focus. This is not a critical problem when the material is large hand-writing on acetate, but when used with a photo copied page from a text book with small text and detailed images, this image distortion reduces the likelihood that students will be able to read smaller print.

Keystoning can also be a problem when slide or video projectors are located too far above or below the screen centreline. Many video projectors offer an electronic keystone correction feature, but this is accomplished by "squeezing" the image into a trapezoid shape before it's projected, pre-distorting it to cancel out the geomteric distortion. The problem with that approach is that you lose real resolution as part of the image is squeezed into fewer pixels of the display. In an XGA display that is 1024 pixels wide, the squeezed edge may only fill 900 pixels of the display. The image shape is fixed but fine detial can be lost in the process. The electronic keystone correction also cannot compemsate for a loss of optical focus so part of the image may be "softer" edged and slightly out of focus.

Many new video projectors offer more significant lens shift to correct vertical keystone problems, and some even offer horizontal lens offset to allow the image to be re-centred on a screen when the projector can't be located on centre.

Careful analysis and design of projection lines and projector and screen locations prevents keystoning from being a significant problem.

Undistorted Image

Keystoned Image