P3D Re: VM and Kids, inter

[Bruce Springsteen <bsspringsteen@xxxxxxxxx>]

Grant Campos pursues what I was originally alluding to, and what Abram
K. deliberately vaguely termed "usable lens diameter", which term Paul
T. also was curious about.  Abram K. then pointed out that it is
actually a very tricky and uncertain calculation given the
characteristics of different lenses, focal length, eye relief (whether
eyelashed or Neanderthal), etc.

In my little experiment, restricted to the VM viewer, I observed that
with my 70mm interpupillary I could still just see the full images
abutting the outer edges of the circular lens openings.  It was
evident that if my interocular magically grew beyond 70mm just then, I
would immediately begin to lose image.  Therefore, I concluded, 70mm
was the outside interpupillary limit for that viewer, assuming one
wouldn't cram the viewer closer to their eyes than was necessary to
see the entire image (except a small child maybe ;-).  I reasoned that
the inside limit would be as far to the inside of the lens centers as
my maximum was to the outside, giving a practical range of 60 to 70 mm
interpupillary.  Didn't think about eye movement, as there didn't
appear to be any difficulty in examining all parts of the picture.  I
assume with all things involving human vision there are a lot of fuzzy
variables and psychological unknowns, so I'm hesitant to ask for hard
and fast formulae to determine the interpupillary limits of any given
viewing system.  But practically there must have been *some*
calculation made by the designers of these viewers, to form a general
idea of who would be able to use their gadgets.  I'm thinking it was a
rough and practical approximation, based on the kind of simple
geometry illustrated in Ferwerda, pp 44-45, and the kind of simplistic
but practical observation I just described.  But I didn't observe
beyond the VM viewer in my hands, so the situation may indeed change
greatly with shorter FL, larger diameter lenses, smaller far point
separation, and what have you.

The question seems to have some urgency for those with especially
small interpupillaries, adults or children, as they are likely to
experience more discomfort or inability to view with the common
systems.  I would be interested to hear Paul Talbot's experiences in
this regard, as he has mentioned this difficulty from time to time.

Somewhat parenthetically, I notice the following in Ferwerda's book:

"If a stereoscope should be adequate for most cases, interpupillary
separations of 50 to 80 mm should be taken into account.  Smaller
values, as occur with young children, are *not important* for
stereoscopy." (my emphasis added)

Bruce Springsteen (I confess, I pushed Humpty Dumpty! ;-) )



  
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