T3D Tom's 1.2 mm question

[fj834@xxxxxxxxxxxxxxxxxxxxx (Dr. George A. Themelis)]

Tom asks if the 1.2 mm "recommended maximum" on film deviation
is perhaps better expressed as an angle 1.2/FL where FL is the focal 
length (of the recording lens, I assume).

One question I have is this:  What came first, the 1.2 mm or the
Realist?  I believe, Seton Rochwite, the inventor of the Realist,
designed the camera with 1.2 mm "aperture offset" (the centers 
of the film apertures of the camera are offset 1.2 mm from the
centers of the lenses).  This creates a stereo window placed at
approximately 7 feet from the camera (when viewing *unmasked*
film chips).  Why did Seton choose 1.2 mm?

The significance of the 1.2 mm comes in projection.  Stereo
projection is usually set so that (1.2 mm) x Magnification =
65 mm (or 2.5").  This is done so that the eyes would not have
to diverge (as they would if this product was larger than the
eye spacing).

Larger film formats can tolerate more deviation simply because
they require less magnification.  Based on that, it is usually
better to use a ratio by dividing the deviation by the width
of the film format.

Things are different in a viewer.  The issue is not that the
infinity separation of the images will exceed the 2.5" eye
spacing but how much convergence can it be tolerated.  Too
much deviation will require that the eyes converge quite a
bit, while maintaining a more-or-less fixed accommodation.
This can be uncomforable for many people.  I do not know what
is considered to be the limit for this.  Stereo enthusiasts 
through years of excercise should be expected to have wider
tolerances that the general public.

It is the in case of the viewer that the ratio of maximum
deviation over *viewing* lens FL, makes sense.

George Themelis


------------------------------