P3D Re: VM and Kids

[Bruce Springsteen <bsspringsteen@xxxxxxxxx>]

So, as Gabriel has pointed out, George is not questioning whether
"kids" have the capacity for stereopsis.  After a certain age, (maybe
by 5 or 6?), most of them do.  The questions he has actually raised,
if I get it, are:  What is the smallest interpupillary separation a
person can have and still see both images in a ViewMaster viewer?  And
at what age do "kids" typically achieve this minimal eyeball separation?

If I read my Ferwerda right, 2 simple convex lenses of a given size,
set in a stereoscope with their principal axes (centers) at an
interocular separation matching the infinity separation of the
stereoview, will allow an unimpeded view of left and right frames for
persons whose eyes are within a *range* of interpupillary separations.
 What is this range?

Well, a VM viewer has an interocular of 65mm, which matches the
infinity separation of the pictures.  Someone whose pupils are 65 mm
apart will look right down the middle of those lenses and see images
*centered* in each circular lens opening.  Someone like me, with a 70
mm interpupillary, will also see two complete images.  However, I am
not looking through the centers of the lenses, but through two points
approximately 2.5 mm to the outside of the lens centers.  The left and
right images, while in the center of my eyes' field of view, will
appear shifted toward the outer edges of the circular lens openings. 
If my eyes grew gradually farther apart, I would bit by bit be blocked
from seeing the outer parts of the frames, until at last I was not
looking through the lenses at all and was seeing nothing.  So my eye
separation of 70 mm is at or near the outer limit of what View Master
optics will allow.  Presumably, a person with an interpupillary of 60
mm, looking through the inner halves of the lenses, would still see
full frames in the center of their field of view, but shifted 2.5 mm
inward, grazing the *inside* edges of the circular lens openings. 
Persons with progressively smaller interpupillaries would gradually
lose the *inner* parts of the frames, until eventually they weren't
looking through the lenses at all. 

So for the sake of argument, lets say the VM system allows
interpupillaries between 60 and 70 mm. (In reality it may be more.) 
According to Ferwerda (as Gabriel said), that encompasses about 90% of
the adult population.  If the real range is more like 58 to 72 mm, the
included group goes up to 98%. This places the VM system exactly on
the middle of the interpupillary distribution for adults, 65 mm being
the middle of the curve.  Not bad for a system originally designed for
adults, as George pointed out.  But what about the kiddies?

We need to do three things, I think.
1.  Determine the actual smallest interpupillary for which the VM
system will allow full frame viewing.
2.  Measure our little tykes' interpupillaries.
3.  Give Viewmaster only to kids whose eyes have the requisite
separation, *then* watch to see if they show signs of recognizing
stereopsis.  (Oooh, Big Bird is sticking his nose at me!)

One last thing.  I believe adding an interocular adjustment to VM,
while retaining the same infinity spacing on the views, would make the
wee ones diverge - and that hurts, mommy!

Happy NSA!
Bruce    



 
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