Optical and Physical lens separations

[P3D <PgWhacker@xxxxxxx>]

>
>George T. wrote:
>>Even if the viewer was adjustable to 50 mm, you still
>>would not be able to see the stereo image because the images 
>>are separated by 65 mm and your eyes would have to diverge. 
>
>The 65mm minimum is true for some formats, but conceivably one 
>could build a 2x2x2 viewer that adjusts the image separation 
>along with the interocular distance down to ~51mm and my eyes 
>wouldn't have to diverge.  

 Ferwerda has a chapter on this.  With diagrams.

  It's worthwhile understanding that a stereoscope's lenses have two 
kinds of separations.  Physical and optical.  The _physical_ separation 
is the distance between the physical centers of the lens chips.  The 
_optical_ separation is the distance between the _optical_ centers of the 
lenses.

    In many old Holmes stereoscopes, and in modern $4 Added Dimension 
lorgnettes, the optical center of each lens lies outside, 'lateral to,' 
the physical center.    In these stereoscopes the physical lens are not 
made from the optical centers of a round lens, instead they are 
(optically) chips taken from the edge of a much larger lens.  Because 
they are thicker on their lateral edges than on their medial edges, they 
are often called 'wedge' lenses, though I suppose the term would offend a 
purist.  They aren't really wedged, they are simply cut (conceptually) 
from the edge of a larger lens.

   At any rate, this is an extremely clever and subtle setup:  it lets 
anyone, provided their inter-eye distance fits within the broad limits of 
the physical separation of the two lenses,  view stereographs with much 
wider than humanly possible image separations -- up to the _optical_ 
separation of the two lenses.  Without having to diverge their lines of 
sight.  




Greg Kane
Denver

PgWhacker@xxxxxxx





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