Star D & lenses

[bercov@xxxxxxxxxxx (John Bercovitz)]

I think it's a great idea to change lenses in the Star D.  
I've been meaning to for some time now.  I bought some lenses 
from Edmund a while back for this purpose.  I'd like to 
comment on the idea of variable interocular, though.  

Variable interocular is sort of a kluge forced on viewer 
manufacturers by the short lenses required to view 35 mm 
format stereo.  Short lenses have very high curvature, (and 
therefore also small diameter).  One must look more or less 
right through their centers to get good resolution because 
the high curvature areas have large aberrations and there 
isn't much diameter to start with anyway.  Since people's 
interpupillary spacing varies widely, short lenses must have 
adjustable interocular.  

Ideally, lens spacing would be fixed and the same as the 
infinity homologue spacing of the stereo pair.  Let me try to 
ASCII why this is so:


      |  |  |  |   <-  rays exiting  ->   |  |  |  |
      |  |  |  |       the lenses         |  |  |  |
      |  |  |  |                          |  |  |  |
      |  |  |  |                          |  |  |  |
      |__|__|__|                          |__|__|__|
      \________/      <- lenses ->        \________/

        \  |  /                             \  |  /
         \ | /                               \ | /
          \|/                                 \|/
    _______.___________________________________._______
   |___|_________|______________________|__________|___|

                            ^
                            |
                  mounted stereo pair
                  with infinity homologues
                  represented by dots

    Lens spacing same as infinity homologues' spacing

Notice that no matter where you put your eyes in the beams of 
rays exiting the lenses, your eyes will see the object 
points' direction as infinity if the lenses are focused to 
give parallel rays.  

Interpupillary distances vary widely but 58.5 to 68.5 covers 
90% of Caucasian males according to a book I have.  I've also 
heard 65 mm is the average.  Regardless, the good area of the 
lens to look through has to be pretty large if the lenses are 
fixed.  This is easy if the lenses have long focal lengths as 
in a Holmes viewer and just possible in medium format but not 
possible in 35 mm format if you want to maintain some 
semblance of "ortho-ness".  What the designer did with the 
Star D was make the lenses much longer than would be correct.  
They're 50 mm and the cameras are only about 35 mm.  Even the 
red button is a compromise at about 43 or so mm.  Not that 50 
mm is a bad choice; it's good if you're shooting SLR/slidebar 
with a normal lens.  So my plan was to substitute good 50 mm 
achromats and use the Star D only for SLR shots.

John B


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