Lens Shift: How one simple projection feature can save time and money

Both residential and commercial installers face challenges when designing spaces using audiovisual equipment. It would be fantastic if rooms could be designed from strictly an audiovisual point-of-view " it wouldn't be so difficult to have great image and sound quality. But of course, architects, interior designers, engineers, and ultimately, the people who will be using the room have many other considerations besides the placement of such equipment. Most rooms have more than one purpose and even if they don't, people usually don't want the technology in the room to be extremely obvious or in the way.

In an ideal display environment, a projector would be perpendicular to and centered to the screen. This of course isn't realistic because most of the time that would place the audience (or something else) between the projector and the screen. Most of the time the projector is mounted to the ceiling, or in some higher location, and projects downward onto the projection screen.

When a projector is not aligned perpendicularly to the screen, the projector must be tilted to reposition the image back onto the screen. This causes an effect called keystoning, where the image becomes trapezoidal instead of perfectly rectangular. Vertical keystoning occurs when the projector is higher or lower (and must be tilted up or down) than the screen and causes a trapezoidal image that is narrower at the top or the bottom. Horizontal keystoning occurs when the projector is tilted right or left and causes the image to be narrower at the left or the right end and occurs when the projector is either further left or further right than the screen. The farther the projector is out of alignment, the worse the keystoning becomes.

Keystoning has traditionally been corrected digitally. When a source is connected to a digital display, such as an LCD projector, an algorithm can be added to the scaling in the projector that alters the image before it reaches the projection lens, correcting the keystone effect. Basically, the algorithm adds compression to the fat end of the image trapezoid so it matches the narrow end. If an image is 80% narrower at the bottom, the algorithm will compress the top of the image's pixels so it too will only take up 80% of the space, adjust downward until the image is a perfect rectangle again.

However, digital keystone correction has some serious down sides. The correction reduces the apparent resolution of the image and can cause a dimming effect, particularly around the edge that is most compressed. It can also add artifacts and other distortion, most notably a fuzziness (lack of sharpness) around the area of the image being scaled most heavily, since it is now no longer the same size as the native resolution of the display. When the image is scaled, you are in effect giving up a part of the display, which includes not only resolution, but also brightness.

Figure 1: Digital keystone correction digitally alters the image before it reaches the display. The dark areas of the chip indicate the portion of the display not being utilized because of the correction, reducing image resolution and brightness.