Re: Stereoscope optics question

[P3D john bercovitz <bercov@xxxxxxxxxxx>]

Message-ID: <199701112215.OAA23006@xxxxxxxxxxx>

Greg Kane writes:
 
>  I've got several stereoscopes of various designs, and I need a 
> bit of help with how the optics work.  Specifically, since I use 
> the scopes for my own snaps, I'd like to know how to calculate 
> the maximum infinity point separation for mounted prints.
 
Out of consideration for those who can't diverge their eyes, one 
should set the infinity point separation so that the eyes are 
parallel when viewing them or very slightly converged.  To avoid 
keystoning, the infinity point separation should be somewhere near 
the distance between the optical centers of the lenses.
 
>    Holding a scope up to the sky and using it to focus the sun 
> on a piece of paper lets you measure the scope's focal length.  
> The other thing you can measure is the distance, on the paper, 
> between the two focused suns. 
 
Excellent and simple tests which you should do.
 
> ? Am I correct that this between-the-suns distance is also the 
> physical distance between the optical centers of the two lenses? 
 
That is correct.  It is not, however, IPD, the InterPupillary 
Distance between a person's eyes (obviously).
 
> ? Is this also the maximum infinity point separation for mounted 
> prints used with that scope?
 
So that a person will never have to diverge his lines of sight, 
yes.
 
> ? Does the maximum infinity point separation differ from the 
> *ideal* IPS?
 
Yes.  The maximum infinity point separation matches the separation 
of the lenses' optical centers, not the IPS of the individual 
using the viewer.  The closer the separation of the lenses' 
optical centers is to the IPS of the individual, the better the 
resolution because the resolution looking through the center of a 
lens is better than the resolution looking through its margin.  
However: 1) What drives the separation of the lenses' optical 
centers is the size of the photos.  Large photos require large 
separation (or else they'd overlap! 8-).  Because the images have 
large separation, so do their infinity points.  The optical 
centers follow the infinity point separation. 2) Longer lenses 
have less aberration in their margins, for the lens diameters 
we're talking about here (big enough for the range of IPDs to look 
through).
 
Here's a person looking downward through a stereoscope (where's 
Honest gAbe when you need him?):

                |<--- ~65 mm --->|
 
 left eye       O                O       right eye
 
           | | | | |          | | | | |  emerging parallel rays
           _________          _________
           \_______/          \_______/  lenses

             \ | /              \ | /    diverging inf. point rays
              \|/                \|/
           ____.____          ____.____  photos

               |<---- ~3.5"? ---->|

As you can see, the eye can be placed anywhere in the emerging 
rays' field (except that some parts of the lenses have higher 
resolution than others).
 
> For the widely available Added Dimension lorgnette viewer this 
> distance measures about 4 inches -- ? Is that the maximum 
> infinity point separation for this viewer?
 
I don't know this viewer, sorry.  (I lead a sheltered life.)
 
John B
 


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