P3D Re: VM and kids, VM interocular, Ideal stereocamera, info needed for new 3D camera

[abram klooswyk <abram.klooswyk@xxxxxx>]

A part of these discussions brings back nostalgic memories to me, for
it recalls my discussions with Koo Ferwerda in 1974 and 1975, and 
later when he was writing his book. As quoted in his book (page 220)
the interpupillary distance figures came from a literature research
I did before the first international stereo congress, Wageningen, 
The Netherlands 1975, where the I.S.U. was founded. 
Koo Ferwerda had send out an international questionnaire to ask for
the properties an ideal stereosystem should have (system: camera,
mounting, viewer, projector and anything other you would like).
In the answers several people stated (experts among them) that the
stereoscope interocular should be "adjustable to the eye base", 
"adjustable for interpupillary variations" or similar statements.
As recently again pointed out by Bruce Springsteen and George
Themelis there is no need for such an adjustment in VM viewers
because the infinity separation (more accurate: far point separation)
is standardised by VM. 
If David Brewster had fixed a standard separation for the optical
axes of his stereoscope (and consequently for the views) it would 
have saved us a lot of trouble, but Brewster was the first who made
the error to adjust the lenses to the interpupillary. 
The first to clarify the optical facts was the German Herman von 
Helmholtz, but he had no influence on the production of 
stereoscopes and views world wide.
I had send the paper "Statements on the Stereoscope" to the members
of the technical working party of the Wageningen congress. 
Among them were W.C.Dalgoutte, Maarten Niermeyer, John Taylor, 
Pat Whitehouse, Fritz Waack, F.Prinsen, Paul Wing, Harm Elsinga, 
J.Bibby, Wim van Slooten and Koo Ferwerda, several of them are now
deceased.
I (among other things) reviewed the history of the opinions on the
interocular adjustment since Brewster, and from the then published 
figures on interpupillaries was clear that a standard far point 
separation and reasonable large viewer lenses were needed with 
no adjustment, in fact also no focusing. I recommended lenses that 
could be used by people with interpupillaries from 50 to 80 mm, 
this means a usable lens diameter of 15 mm, not including the edges.
Most VM viewer lenses have less then 10 mm usable diameter.

Indeed the usable inner distance of viewers lenses limits the range
of kids that can use VM viewers, I estimate it to be not under 53 mm, 
very young and small children cannot see stereo with those viewers. 
As was clear from literature abstracts I posted earlier, infants have
measurable stereopsis from 2 to 3 month of age. There seems no reason
to bother them with VM views at that age, but a little later
grandfathers might try, even when the infants interpupillary still
is too small. 
The solution obviously is: cut off the lenses and replace them by
larger ones, preferably with a little smaller focal distance, or buy
such an adapted VM viewer, as I did (not for a grandchild). 
Now as theory tend to work only partly in practice, I also DO have
a "Focusing VM" viewer, but that one also has shorter focal 
distance, which gives a wider viewing angle.

The best way to use a stereoscope is to have the pictures in the
focal plane, so that parallel rays come out of the lenses and you
can look without accommodation. For normal sighted people this 
means that focusing should be done by moving the lenses as close
as possible to the pictures (keeping them sharp of course). 
Then you look at far points without accommodation and parallel 
eyes axes. 

When this would be done by everyone the theory would work, but
unfortunately people KNOW that the views are very nearby. 
Many of them therefore unconsciously converge (squint) and 
accommodate when looking in viewers. They of course try to adjust 
focus and eventually interocular, sometimes getting fusion and 
sometimes not, several with eye strain and probably divergence.
This seems the mean reason why some people, with otherwise normal
stereoscopic vision, fail in using some viewers.
Try to convince them that the stereo scene is deeeeeeep.

Abram Klooswyk


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