P3D Re: Keystone

[Brian Reynolds <reynolds@xxxxxxxxx>]

Dr. T wrote:
> For example, you asked about "keystone distortion".  It helps if you
> know the shape of a keystone or if I could draw an ASCII drawing!
> 
> 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.  If you go to the right of it then the opposite side
> will be larger.  Hence the right and left images do not match and you have
> vertical misalignment in the stereo pair.  This will happen if you converge
> the camera.  If you keep it parallel, you don't have this problem.
> 

This is also known as convergence.  The best (non-stereo) example of
vertical convergence is trying to take a picture of a tall building
from near its base by pointing the camera up.  The top of the building
will be recorded on film as much smaller than the base, and your
perception will be that the building is falling backwards.  This is
caused by the different projection distances from the top of the
building to the lens and the base of the building to the lens.  With a
view camera you can correct this by tilting the camera back (if you've
pointed the camera up to get the top of the building) and changing the
projection distance from the lens to the bottom and top of the film.

Humans don't have as much of a problem with horizontal convergence
(which is why two point perspective works for drawing and painting).
The problem with horizontal convergence in stereo photography is that
the different camera positions and angles (when using toe-in) cause
conflicting convergence (i.e., different vanishing points).

> BTW, Pennsylvania is known as the Keystone State because it
> resembles a Keystone and the company Keystone got its name from PA.
> 

I was under the impression that Pennsylvania chose its nickname
because of the function of a keystone, not its shape.  The keystone of
an arch is the center stone against which both halves of the arch
press.  Pennsylvania was the geographic center of the 13 states when
the US declared independence.

-- 
Brian Reynolds                  | "Dee Dee!  Don't touch that button!"
reynolds@xxxxxxxxx              | "Oooh!"
http://www.panix.com/~reynolds  |    -- Dexter and Dee Dee
NAR# 54438                      |       "Dexter's Laboratory"


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