Re: Effects of Eye Position on Stereo Perception

[P3D John W Roberts <roberts@xxxxxxxxxxxxxxxxx>]

>Date: Fri, 18 Apr 1997 05:16:05 -0500
>From: P3D Peter Homer <P.J.Homer@xxxxxxxxxxxxxxxxx>
>Subject: Effects of Eye Position on Stereo Perception

>This topic would seem to be relevent to this group when it concerns stereo
>perception. The eyes converge on objects they are viewing and it was quite
>a logical early view that this convergence gave information on depth by a
>kind of triangulation. But I had a photocopy from a book on psychology that
>stated that that theory and experimentatation showed that convergence of
>the eyes played little or no part in depth perception . 

While neither field seems to have produced a really consistent and 
comprehensive model of how it all works, I would tend to trust the reports
of vision researchers more than the reports of psychologists on how the
"mechanical" features of vision such as depth perception work. And all the
vision research works that I can readily lay hands on agree that both
convergence and position on the retina play a role in stereo perception.

>Stereo viewing with
>stereoscopes and particularily free viewing would seem to support this .
>Although perhaps because of the belief that convergence was neccessary for
>stereo perception it seems it was originaly thought that convergence was
>neccessary in a stereoscope as well.
> I have seen a diagram of Wheatstones stereoscope from above which show the
>viewers eyes converging on a point behind the mirrors. Apparently his and
>later Brewsters reason for using half lenses originaly was not so much to
>diverege the lines of sight out to pairs larger than the interocular. But
>to diverge them on the other side of the lenses so that they converge on a
>point between the pairs.

Well, the eyes do converge, in general. Also, you've left out about a century 
of development between the earliest stereoscopes and the Realist-type slide
viewers. Much of this advance went into systems using Holmes stereoscopes.
The earliest stereographs had serious problems by current standards - by the
time of the Civil War they were greatly improved.

>  Modern stereoscopes for 35mm slides are not constructed with half lenses
>and unless what happens with those with adjustable eyepieces is that we
>actualy set them so that we are looking through the inner halves of the
>lenses rather than the centre as we would think . What about those with
>large fixed eyepieces would only those people with the right eye seperation
>to see throught the inner halves be able to see a proper stereeo image?.

I recommend that you read "The World of 3-D" by Ferwerda, to correct this
misimpression. The half lenses or looking through the inner halves of the
lenses are irrelevant. The object in the design is to have the spacing between
the optical axes of the lenses equal to (or slightly greater than, to allow
for variations in manufacture) the infinity separation of the stereo pair
being viewed. The classic stereo cards have infinity separation of something
approaching 3.5", and since very few people have an interocular distance
that large, it is natural that most people would be looking through the inner
halves of the lenses. You don't have to use half lenses, but cutting one
lens in two saves on glass, and assures that both sides will have the same
focal length - it's much easier and cheaper to make a nice uniform lens than
it is to make one with a precisely controlled focal length, especially with
the older lensmaking methods. Stereoscope makers sometimes say that their
lenses have some "prism" build into them, but that's just shorthand notation.
Slide viewers generally use small slides (<<3" across each slide), so the
infinity points of the two slides are not placed further apart in the mount
than the typical adult interocular. Having an adjustable interocular in a 
slide viewer actually hurts the fidelity of reproduction of the stereo effect -
it's the compromise chosen to compensate for having little tiny lenses. 
The need for adjustable interocular is a bug, not a feature.

>With parrallel "free viewing"  we can be even more certain that our eyes
>are not convergent 

Very, very wrong. When you free ("parallel") view a stereo pair, your eyes
*do* converge over most of the depth range of the image. Only at a point
somewhere near infinity do your eyes actually not converge, but that's also
true of looking at the real world without a viewer.

Your eyes don't converge *at the distance of the actual stereo pair*, but
at a point somewhat further away (as you described for the earliest
stereoscopes), the degree of convergence of the eyes being a function of
the apparent distance to the part of the image you're looking at. The eyes
converge when using a Holmes stereoscope, and when free viewing, and when
using a Realist-type slide viewer.

When viewing stereo pairs, there's a certain degree of decoupling of the
correlation between convergence and focus (accommodation) that is used in
normal vision. That's actually a health concern, and one of the reasons that
it is recommended that people viewing stereo images "take it easy", not strain,
take rest breaks, and so on.

>but we see a stereo image and as our brain has not
>evolved for viewing stereo pairs but real objects it is likely that this
>mechanism is involved in the viewing of such real objects. 

But the brain quickly learns to set aside certain inconsistencies in viewing
stereo photographs (at least among those of us who enjoy viewing them).

John R


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