THE BIG PICTURE

Keystone Kops

Scott Wilkinson  |  Apr 8, 2009

I am building a dedicated home theater, and I have completed most of the design work. One of the last details is the projector position. I have a 100-inch 16:9 screen and the Panasonic PT-AE3000.

I have read that it is better to shift the image optically rather than using keystone correction, so I am considering having the projector mounted lower from the ceiling. How important is it to reduce/eliminate keystone versus image shifting? Should I position the projector so keystone is zero? Also, how important is the distance between the screen and projector? Currently, the plan is within the recommended range of 10 to 19 feet for a 100-inch screen.

Charles Ballaro

First, let me clear up any confusion about the function of keystone and image shifting. Keystone is a setting found in most projectors that compensates for non-perpendicular placement. If the axis of the projector's light path is not perpendicular to the screen, the image will not be rectangular—one side will be taller than the other and/or the top or bottom will be wider than the other. Keystone correction compensates for this by digitally processing the image to form a rectangle.

By contrast, optical image shifting (sometimes called lens shifting) simply moves the entire image up, down, right, or left to align it with the screen. This lets you place the projector off-center with respect to the screen and still align the image with the screen. In this case, if the light path is perpendicular to the screen, no keystone correction is required.

You should absolutely avoid keystone correction at all costs. The digital processing it employs reduces resolution and can introduce ugly artifacts. You should mount the projector so its light path is perpendicular to the screen. Then you can use lens shift to align the image with the screen.

As for the distance between the projector and screen, which is called the throw distance, it's better to place the projector as far as possible from the screen. This allows the light to pass through a smaller area in the center of the lens, resulting in less potential for chromatic aberration.

If you have an audio/video question for me, please send it to scott.wilkinson@sorc.com.

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