T3D Re: A Third Simple One

[john bercovitz <bercov@xxxxxxxxxxx>]

I wear tri-focals and have only three rather limited ranges of 
sharp vision.  I have only slightly more than 1/4 diopter of 
range left.  When you consider that the normal eye is stopped 
down enough to give approximately 1/4 diopter for free, you see 
how very little accommodation I have.  Yet my limits on what is 
an acceptable level of deviation seem to be no different from 
other folks'.  Caution: one should not generalize from a specific 
case to the whole.  

My guinea pigs are my friends who come over to view slides in my
little screening room.  I show them slides I think are easy and
they think they are easy.  I show them slides that are marginal 
and they think they are marginal.  These guniea pigs are sometimes
members of the local stereo club and sometimes are complete newbies.
The demographics of our stereo club are such that the average mem-
ber's age is only slightly less than Yoda's when he died so I 
imagine their accommodation is down to only a couple of diopters.  
The newbies I've tested tend to be young folks, say 20 years, and 
they have the expected accommodation of their youth.

I prefer to think in terms of angles, that is, maximum disparity 
of 1/30th of the focal length in an ortho situation is my limit as 
well as theirs for difficult scenes.  1/15th is a stretch but we 
all seem to be able to do it if the scene is easy.

John B

PS: Of course, testing people is how I got really interested in maofd.
My series of test slides, one of which is in that article in 
Stereoscopy, was shown to a lot of people.  No one had trouble viewing 
it.  The lens focal length was 50, the stereobase was 192 mm and the 
distance to the objects was near of 1.8 m and far of 2.2 m if I recall 
correctly.  1.8 m / 192 mm = 1 in 9 rule in conventional terms.  Once 
I found that no one had trouble viewing that slide, I became a real 
believer.  


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