What is Keystoning?

If you have video presentations of any kind at your meetings, you will eventually run into a problem called “keystoning” or the “keystone effect”. Taken originally from architecture, keystoning is the name given to the effect that occurs when the projector is placed above or, more commonly, below the center for the projection surface (the screen), which results in the image hitting the screen at an angle. If the projector is placed below the center of the screen, then the resulting image looks like a keystone, with the top of the image wider than the bottom.

So how can you fix this?

Placing the projector centered relative to the screen and exactly perpendicular to the screen will solve this but, while it is the optimal solution, it is often not an easy solution to implement. Fortunately, projector manufacturers have two possible technical solutions to the problem and at least one of these will be built into most modern projectors: Digital Correction or Optical Correction.

Digital Correction, or Keystone Correction, is very common with LCD projectors and is easy for the average person to use. In fact, there are often buttons on the projector that allow the user to correct keystoning without needing to go through sub-menus. What digital correction does is compress the image on one side (be it top, bottom, left, or right) while expanding the image on the opposite side. This “squares off” the image, resulting in a box shaped image. A word of caution, though… If the correction goes too far in compressing or expanding, some images could look a bit distorted at the edges of the projection. This is rare, but I have seen it happen.

Optical Correction, also called Lens Shift, physically adjusts the position of the lens to square up the image. This often results in a clearer image than digital correction but is a high-end feature so is typically only available on the more expensive LCD models.

Most of the planners I know find digital correction to be sufficient for their needs but those working with detailed video projections prefer to use optical correction. I also find that, for smaller screens and smaller groups, digital correction works just fine. For larger events, though, where I need a much larger image, getting a projector with optical correction gives me the extra quality that I am looking for. Then again, if the image quality is that important, I will also go to the effort to raise the projector to be closer to the center of the screen. After all, why worry about keystone correction through the projector at all if I can simply move the projector itself and avoid the problem…?

~ Karl Baur, CMP • Project Director, RDL enterprises