Re: PePax and dollhouse effect

[P3D John Bercovitz <bercov@xxxxxxxxxx>]

> If I take my SLR and take a shot of a subject from a given
> distance, all other shots of that subject from the same distance
> will have the same x,y scale. 
[..]
> Other distortions enter into the situation but a linear
> shrinking in all axes just doesn't make sense.
 
If you take all shots from a given distance, then the on-film 
size of the image will always be the same.  But that doesn't
mean the reconstructed size will be the same.  For instance,
if you take a shot of an 8' square object from a distance of
20', and then take a shot of a 4' square object from 10',
the two will have the same on-film size.  That's the sort of 
thing that is operating here.
 
If you have web or ftp access, you can go to bobcat.etsu.edu and 
change directories to pub/photo/photo-3d/technical and pick up 
orthomag.gif and orthosep.gif.  These will show how the image 
reconstruction is done.  The basic process is to first create the 
image on the film by tracing lines from the object through the 
(pinhole if you like) lens onto the film.  Next you take that film 
(now a transparency) and view it by tracing lines from the eye 
back through those points on the film until they intersect and 
reconstruct the object.  If Bob's gotten a chance to move it to 
the technical directory, there is also something called PePax.gif 
which shows this process for a doubling of lens focal length and a 
doubling of stereobase.
 
John B
 


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