Re: Ortho Seat

[P3D John Bercovitz <bercov@xxxxxxxxxxx>]

> In any event, the ortho seat is placed at a distance from 
> the _window_ that depends on the ratio of the camera film 
> aperture to the _camera_ lens focal length.

I disagree (Is that OK?  8-)  I'm going to try an off-beat,
non-perspective-oriented, proportions argument.


       infinity             infinity

           ^                    ^
           |                    |  
           |                    |  
           |                    |  
           |                    | 
           |                    | 
           |                    | 
           |                    | 
           |                    | 
           |                    | 
           |                    | 
           |                    | 
           |                    | __________   window
           |         /\         |         |   distance
           |        /  \        |         |
           |       /    \       |         |
           |      /      \      |         |
           |     /        \     |      2100 mm  (~ 7')
           |    /          \    |         |
           |   /            \   | ______  |
           |  /              \  |     |   |
           | /                \ |   36mm  |
           |/                  \| ____|___|_
          left                right   
          eye                 eye

           |<----- 65 mm ------>| 



36 mm is the distance from the eye to the slide when
you view the slide in an ortho hand viewer.  2100 mm 
is the distance to the stereo window.  The slide is 
24 mm high and by proportions, the stereo window is 
(24/36)2100 = 1400 mm high.  

If you project the images onto a screen so that the
images are 1400 mm high, then you need to sit 2100
mm from the screen.  If you project the images so that 
they are 4200 mm high, you still need to sit 2100 mm
from the stereo window (that's a fixed number because 
it is the distance to the stereo reconstruction of
the window), but you are now 6300 mm from the screen.

So whether you are 36 mm from the image, or 2100 mm
from the image, or 6300 mm from the image, you are
sitting 1.5 times the image height from the image.

John B


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