P3D DVG 'pinhole spectacles'

[Peter Abrahams <telscope@xxxxxxxxxx>]

As a nearsighted person, for years I've made 'pinholes' by squeezing two
thumbs & forefingers together, to sharpen a distant view when I didn't have
my glasses.  Every once in a while, a post to a vision newsgroup asks about
pinhole spectacles.  These aren't exactly what DVG uses, but close.  They
sharpen the view considerably, at the expense of field of view; in the same
way that stopping down a lens increases depth of field.  I've never heard
of someone using them to enhance stereo, and fabricated a pair of pinholes
to try the DVG approach.   I taped aluminum foil to a card, punched a
round, 1/16 inch hole, managed to flatten the foil while maintaining a
round profile, blackened it, and used it to view.  DVG's instructions
require a precise horizontal placement, where only the inner half of the
field of view is shared by both eyes.
Pinholes are known to greatly enhance depth of field, to the point that a
nearsighted person can use them to see distant objects.  They also greatly
blur the edges of the field, from diffraction effects.  Only the center
area of the field of view is sharp.  A very round hole will minimize this
effect.  I found that using two pinholes for two eyes gave a variety of
effects because of this, by moving the pinholes together & apart, the two
focused areas in the middle of each hole came together & then moved apart.
The diffracted edges interacted in a very interesting way.
I wonder why pinhole photographs are as sharp as they are, since for visual
use the outer half of the field is not sharp at all.

I did not seem to possess the ability to see a much-enhanced sense of depth
in the (mono) photographs I viewed with my pinholes.  I did attempt to
place them according to instructions, and did see a noticeable enhancement
of depth, but it was a minor effect.  I saw it in scenic photographs,
architectural shots, and others, but there had to be some cues to depth in
the flat photo, such as receding lines or layers of landscape.  The effect
was much more dramatic when viewing the real world; the greatly enhanced
depth of field & sharp images gave a good stereo effect.

The sense of depth used by our vision system is such a complex mixture of
responses to cues, that it's tough to give a cause for this effect.  The
chromatic aberration discussed before is a good guess.  Only the center of
the field is 'in focus' with these pinholes, the edges are blurry; and this
effect is similar to standard field curvature (though it is caused by
diffraction).  As Larry Berlin has noted, binocular viewing of an effect
(that I thought was field curvature) can give an impression of a bowl
shaped image, which could add to the DVG effect.  A third cause could be
the pinholes 'projecting' a narrow beam of light from an image, onto
disparate areas of the two retinas (eliminating the surrounding, distant
areas with less disparity).

To sum up, I'm going to keep these pinhole viewers, but not because they
added much depth to flat images.  I can believe that others saw a
significant effect, but I did not.
_______________________________________
Peter Abrahams   telscope@xxxxxxxxxx
the history of the telescope, the microscope,
    and the prism binocular


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