Re: Shift lenses

[P3D William Carter <wc@xxxxxxxxxxxxxxxxxxx>]

Wm. C. writes:

> Keep the film plane parallel to the screen...

John B responds:

>Can anyone explain how this cures keystone?  It's not intuitively 
>obvious to me.

Only the vertical axis of the beam is affected by the wedge prism. The 
horizontal axis is not (the vert. & horz. elements of the image). 
Therefore as the beam is steered through the prism, the further, less 
retarded part (in your case top) of the beam (think of it as swinging 
through a hinge?) will be wider than the bottom... keystoned.

When this keystoned beam images on the screen, it will keystone 
oppositly. That is, the top now is narrower than the bottom. The result 
is a cancellation of these two keystoning effects, rendering a normal 
image. 

In fact I think you'd have a tough time creating a keystoned image with 
any manner of mirrors and/or prisms as long as you kept the film 
parallel to the screen.

I just looked at my Melles Griot price list for '92-'93. Sheese! You 
might think about making a prism out of plexiglass and water!? 

-- 
                wc@xxxxxxxxxxxx
    37deg 39.09'N x 122deg 29.56'W x 90'MLLW




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