[photo-3d] Re: viewer optics. Brewster, Helmholtz, van Albada 1/2

[abram.klooswyk@xxxxxx]

John Goodman   16 Mar 2001:
(...)
>I can't escape my 68 mm interpupillary separation, so it 
>would seem logical, for most comfortable enjoyment of stereo 
>views having a homologous infinity point separation several 
>millimeters less than 68 mm, that some degree of convergence, 
>provided by moving the centers of the lenses somewhat closer, 
>is required. Or not? 
(...)
>If the viewer oculars are set to the infinity separation, and 
>my pupils have a wider separation, how can I be looking at 
>distant points in the stereo view with parallel eye axes?

John,
This is harder to discuss in words than by looking at a diagram.
The schemes which I quoted (Ferwerda etc) are all similar,
but John Bercovitz' new one (at Paul Talbot's site) is the prettiest:
http://www.rmm3d.com/3d.encyclopedia/mounting/ViewerLensSpacing.pdf

Remember that you can burn a hole in a piece of paper with a 
magnifier, by uniting the parallel rays from the sun in the 
focal plane. In reverse, rays from one point (if in the focal 
plane) will come out of the lens as parallel rays. So it 
doesn't matter where the eye is with respect to the lens, it 
will always see the same point in a particular direction.
(Encourage children to play with a magnifier ...)

For stereoscope use this means that when the spacing 
(separation) of the infinity homologues is equal to the lens 
separation _and_ the picture is in the focal plane, there will 
come parallel rays from an infinity image point at any point 
of the lens surface.
So with any interpupillary distance the same parallel viewing 
of points in the distance is achieved.

Vintage views often have infinity separations of 80 to 85 mm. 
The optical centers (axes) of Holmes stereoscope lenses 
classically are at the outer rims of these lenses, which are
halves or quarters of a lens. When the spacing of the optical axes 
would be 85 mm, and the view set in focal plane, it wouldn't matter 
if your interpupillary was 68 or 50 mm, viewing would be the same.

For modern 35 mm slides some people have said that 
stereoscopes should have a fixed focus setting (slide in focal 
plane) and a fixed separation of the lenses. But then all 
slides must have the same separation of far points. 
An ideal not often realized...

Abram Klooswyk 


 

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