T3D re: interocular question

[abram klooswyk <abram.klooswyk@xxxxxx>]

It is sometimes confusing to have two list (T3d and P3d)
Recently (PHOTO-3D Digest 3222, 02 Mar 1999) I wrote:

>>When [Sir] David Brewster first published his stereoscope (1849,
1851) he wrote: "The semilenses should be placed in a frame, so 
that their distance may be adjusted to different eyes". 

In 1867 the famous German physiologist Hermann [von] Helmholtz
(the "von" is the German equivalent of a knighthood, so it is
not in his early writings) published his modification of Brewster's
stereoscope. I translate (:-)) a part of his German text:

"Because the distance of corresponding points in the usual
photographic stereoscope images not always is the same as that
of the eyes (...) should one, in order to achieve the most
natural projection of the objects, have the possibility to
adapt the instrument to every image" (... das Instrument jedem
Bilde adaptiren koennen).
Helmholtz also advocated looking with the visual axes parallel
when looking at landscapes pictures, as Claudet had done before.

A scheme of a stereoscope (seen from above), two corresponding
Far points F on the picture, which is supposed in the focal plane
of the lenses (Sorry, I could find only these rough ones :-)),
so there will come only parallel rays out off the lenses, from 
each point of the picture. Lens separation is made equal to stereo
picture far point separation.
Therefore three persons with Large, Medium and Small 
interpupillary distance all can look with parallel eyes at far 
points in the picture. Only Mr Medium can look though the lens 
centers, the other two use the lenses off center.
         
              .-.                    
            .'   '. --------------------------------L        
|          /       \                 
|         |         |
|F        |         | ----------------M
|          \        /                
|           '     .                 
             \   /  --------S
               '.

              .-.                    
            .'   '. --------S        
|          /       \                 
|         |         |
|F        |         | ----------------M
|          \        /                
|           '     .                 
             \   /  --------------------------------L
               '.
                                                (c) Closearts

This scheme is the viewing method recommended by Helmholtz.

The essential difference between Brewster and Helmholtz is the
adjusting of the lens separation to the eyes OR to the pictures.
When Mrs Small would adjust the above stereoscope "to her eyes", 
to look through the centers of the lenses, she would have to 
_diverge_ (look wall-eyed) to see the far points. 
Adjusting lens separation to the stereo picture's infinity or
far point separation will avoid this. 
(The picture should be kept in the focal plane, for people with 
normal - or corrected to normal - eyes, this can be done by moving
the lenses as close as possible to the pictures, while keeping 
vision sharp)<<

A little longer ago Bruce Springsteen more or less forced me to
a rough experiment which anyone can do (and maybe better), it was
in PHOTO-3D Digest 2883, 07 Aug 1998.

>>OK, I also did some additional primitive measuments on the VM.
With eyes close to the lenses, looking with one eye and head fixed,
I can move the viewer 7 mm from left to  right and still see the whole
VM picture. Moreover, the lens separation is not 65 but 64 mm.
The interpupillary range then is 58 to 71 mm. <<

Holding the head fixed is most difficult in these kitchen experiments.
Please don't quote these values as reliable.

Abram Klooswyk


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End of TECH-3D Digest 461
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