P3D testing stereo vision

[Peter Abrahams <telscope@xxxxxxxxxx>]

Jim's experience with his eye doctor reminded me of some years ago when I
had an ophthamologist with less 'vision' than was needed for my case.  He
was a great surgeon, and a capable practitioner for the masses, but
couldn't handle someone who really used their eyes.  He finally told me
that his tests and my experiences didn't dovetail & I could see a
psychiatrist if I wanted to work it out, so I found another eye doctor, one
that I could work with.

I would expect that simple screening tests for stereoscopy would have
limited relevance to someone who has extensively trained their stereo
vision.  An individual with decoupled convergence & accomodation is not
what these tests (or testers) are prepared for.  You were give 2/3 of the
very rudimentary Titmus test, vectographs that are popular because there
are no instructions needed: if the fly looks disgusting, you have the
skill.  There are more elaborate tests, including random dot stereos.  Many
stereo photo cards were sold by Keystone as 'eye exercises', that included
some tests, and there were also Keystone testing kits for topographic
mapping, used to screen people who were to spend careers looking at stereo
aerial photos and make topo maps.  Other tests, used to screen children,
had cards with animals, cartoons, etc. that could be fused with viewers.
(see sell 3-d for some of those).  One thing these kits are great for is to
'become aware' of the muscles that converge & diverge the eyes; you can
feel them working as you run through these cards.

It seems possible that the stereo viewing skills that Jim has developed
somehow interfered with the test, but offhand I can't see why that would
have made depth in the vectographs more difficult to see.
However, on p155 of Howard & Rogers, _Binocular Vision & Stereopsis_
(1995): "...investigations of correlations between different tests of
stereopsis....the correlation between them was not significant....".  No
doubt there are more advanced tests than those discussed in this reference,
but there seems reason to doubt results from one test if they are at odds
with experience (like being able to see stereo in photos).  For example,
the circle test you mentioned has diamonds in circles, and if you have
strong astigmatism & were tested without glasses, and the axis of your
astigmatism was aligned with one of the edges of the diamond, the loss of
resolution might have an effect.
But as far as the problem with reading that was mentioned, where the right
eye sees lines of text that are slanted, I'd try another spectacles even
though the doctor didn't find anything wrong; correction for astigmatism is
a cylindrical lens that would produce that effect if one lens is rotated,
though this is something that is routinely tested & maintained to some
precision.  Also make sure that fatigure from stereo viewing isn't a factor.

>Could it be that I really do have poor
>stereo vision, and the reason I like stereo images so much is that the
>slightly hyper quality of many such images allows me to see the world in
three
>dimensions as people with normal vision see it all the time? 

I have an interpupillary distance of 57mm, and am sure that someone with a
75mm IPD has enhanced stereoscopy.  The difference of 18mm is the same
proportion as the difference between 75mm and 100mm, and a pair of cameras
with lenses seperated by 100mm would be considered somewhat hyper for most
applications.  So, if stereo photos taken at 100mm separation seem hyper to
some viewers, then the person with 75mm IPD would have a hyper view
compared to mine.  But I'm not sure this means I like 3d photos any more
than Mr. Hyper, and since 'ortho' for the 'normal' person would be slightly
hyper for me, I'd expect that you or I would like less hyper, not more
(hyper for a normal person would be super-hyper for me).

>From another response to this topic; up in the remote reaches of the frigid
north, Bob seems to be associating with chameleons....I'm not sure I've
witnessed this skill:
>Now what's *really* amazing to me are the people who seem to totally
>decouple the eye muscles and let one wander around while "viewing"
>with the other.
_______________________________________
Peter Abrahams   telscope@xxxxxxxxxx
the history of the telescope, the microscope,
    and the prism binocular


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