P3D Re: Keystone

[fj834@xxxxxxxxxxxxxxxxxxxxx (Dr. George A. Themelis)]

>Imagine a rectangular.  If you go to the left of it and point the camera,
>the side of the rectangular closer to the camera will be larger than the
>side further away. 

And of course, you realize how the focal length affects this... If
you are using a wide angle lens and come closer to the subject
then the keystone distortion will be worse.  If you use a telephoto
lens and stay further from the sample then it will be better.

As you use a longer and longer lens then keystone distortion is
eliminated.  In reality, what is eliminated is the perspective.
Things do not vary in size with distances.  This is the case is
high magnfication microscopy as when using the scanning electron
microscope.  The effective focal length of the instrument is 
(working distance) x (magnification).  All SEM stereo pairs are 
taken by pure covergence (tilting the sample usually) but 
keystone distortion is not an issue at high magnifications.

George Themelis


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