Keystone Kops

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.

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Jeffrey's picture

Scott, I thought that the further away you placed the projector the less light output that reaches the screen. Is that not true? If Charles is using a 100 inch screen (width?) would this not be a consideration? I have always mounted mine fairly close (about 11' for an 80" wide screen) to maximize the punchiness of the image. Perhaps I am wrong. Thanks!

ken's picture

Scott, Have you heard of anyone having trouble with TV GUIDE ON SCREEN or finding vcr+ codes?Thanks

ender21's picture

@ken - huh?? Scott, my dishes aren't coming out as squeaky clean as I'd like them to. Do you have any recommendations on dishwashing detergent? ;-)

Scott Wilkinson's picture

Sorry, Ender, that's a question for! ;-) Ken, I haven't used TV Guide On Screen or VCR+ in a LONG time, and I don't recall having any problems with them at that time. Jeffrey, the same amount of light

Fred M's picture

Scott - one small clarification: if you are in fact employing a smoke machine while watching the Rocky Horror Picture Show, then, yes, there is a corresponding drop-off of light output, proportional to the creepiness dial setting on the smoke machine.

Scott Soloway's picture

When using a zoom lens with a variable aperture the light output does indeed vary with the projection distance for a fixed screen size. Light output is higher with more zoom (shorter throw) and contrast is better with less zoom.The amount of variance depends on the projector design.

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