[Don Black]

I'm puzzled by the issue of keystoning. It seems that projectors either need to be aligned with the top or bottom of a screen. Anything in between will require you to tilt the projector and hence experience keystoning. First, if I'm wrong about this, please correct me. However, assuming this to be true, what about the tilt causes the trapezoidal effect? Thanks.

 

[Steve13]

The trapezoidal image is caused because the screen and projector lens aren't parallel to each other. For example, on a ceiling mounted projector that is tilted down, the top of the lens is slightly closer to the screen then the bottom of the lens. Since the lens is closer, the top of the image is smaller and it gradually gets bigger as the distance from the lens to the screen increases, causing the trapezoid effect.

 

[Don Black]

Ah. That makes sense. Thanks Steve. Could you explain why the anti-keystoning tools cause the problems that they do though?

 

[mrtanner]

The electronic keystoning correction can adjust for the angle of the projector to the screen by stretching or schmushing (hope I didn't lose you with the technical jargon) the image on the top or bottom of the screen to keep it rectangular. The primary drawbacks are:


1. The edge that has been made smaller is "wasting" some pixels so total resolution is lower.


2. The light is still striking the wall at an angle from the lens so some "oversplash" or aura will most likely be visible just above or below the picture.


This said, many projectors still provide very nice results with keystone correction, but you are better off if you can avoid using it.

 

[rlsmith]

We should add that an optical lens shift will not cause the kinds of distortions that are caused by digital methods as described in the last post. Shifting the lens changes the image in such a way that it anticipates different distances for the top and the bottom (say) giving a correct final result. Some projectors have both horizontal and vertical lens shifting, allowing the projector to be off-axis horizonally, vertically, or both.


Lastly, many projectors are designed to be off-axis. Such a projector is intended to be aiming from the bottom (or dually from the top) of the screen. The term "projection angle" refers to the angle anticipated by such a projector. A bit of trigonometry allows you to compute where to situate the projector for minimal keystoning. Obviously, if the lens is a zoom lens and can therefore change the focal length, this also has an affect on the situation.

 

[rogo]

FWIW, the Sanyo Z1 can be pretty much anywhere between the top and bottom of the screen due to its lens-shift feature.

 

[ChaCha]

The X1, while a great machine, sucks in terms of versatility of placement. You CANNOT have the projector vertically within the screen area without tilting or keystoning.