Re: Viewer interocular - again

[P3D <PgWhacker@xxxxxxx>]

>I don't think, however, that I can easily translate the technical details
>into the steps that should be used to properly position the interocular
>adjustment lever on a Red Button viewer.


   More about how a stereoscope works. 

  You focus a stereoscope by moving the lens or the stereograph in and 
out until the stereograph lies in a plane that is one focal length away 
from the lenses.  Light diverging from a point on the stereograph is bent 
by the lens so that it's rays come out the other side of the lens 
parallel to each other.  Why that's important is hard to visualize this 
without seeing a diagram.    The practical point is easier to picture:  

    The stereoscope's lenses bend the light so that each of your eyes 
sees its stereo-half as if the stereo-half were moved back to infinity.

    If the optical centers of the stereoscope lenses are separated by a 
distance _equal to_ the infinity separation of the stereograph, each of 
your eyes sees it's stereo-half by looking _exactly straight ahead._

   If the optical centers of the stereoscope lenses are _farther apart_  
than the infinity separation of the stereograph, each of your eyes has to 
toe in, 'converge,' to see its stereo-half.

   If the optical centers of the stereoscope lenses are _closer together_ 
than the infinity separation of the stereograph, each of your eyes has to 
angle outward, 'diverge,' to see its stereo-half.

     Human eyes are designed either to look straight ahead (when you look 
at things in the distance) or to converge (when you look at things up 
close).  On the other hand you can live a lifetime without coming across 
a situation where wall-eyed, diverging, vision will do you any good.  
Diverging your eyes isn't natural.
 
  For stereoscope users the point boils down to this:  you need a lens 
separation that lets your eyes point the way they are designed to point:  
either straight ahead or converged.  Either way is comfortable.   So for 
comfortable viewing, the optical centers of the stereoscope lenses must 
be at least as far apart as the infinity separation of the stereograph.  
Using a stereoscope with  lenses whose  optical centers are farther apart 
than the infinity separation of the stereograph will require your eyes to 
toe in -- which they are designed to do quite comfortably.

    NOTE this critical and counter intuitive point:  it's the _lens 
separation_, not your eye separation that matters here.  If the lens 
separation is right then if you can line up your eyes so each eye looks 
through _any part of_ it's lens, the setup will work for you.  **It's the 
LENSES' separation, not your eye separation, that controls the 
correctness of the alignment.

>Also, Greg's description suggests that when sharing a 50s slide viewer
>with a group, the inter ocular should be set for each slide and left alone
>as it is passed around.  Is that the intended use of the lever?  I had
>always believed that each individual would need to adjust the lever to
>accommodate their eye spacing

   Red Button Viewers:  you use the adjustment lever is there for three 
reasons.  

      1.  Because it's there, and you have to fiddle.

       2.  To move the lenses in front of your eyes.  If the lenses 
aren't in front of your eyes, you can't see through them.

       3.  To adjust the separation of the lenses so that the distance 
between their optical centers is greater than the infinity separation of 
the stereo slide.  Which is why Realist slide chips are mounted a bit 
closer together than the camera's lenses and the average person's inter 
ocular separation -- everyone can converge, few can diverge.

   
So:  as you pass the viewer around, you really only need to adjust it if 
the next person in line can't see through both lenses at once.   You 
don't need to make optical-center/infinity-point-separation adjustments 
for each new user.




Greg Kane
Denver

PgWhacker@xxxxxxx





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