P3D Re: re: Free Viewing

[abram klooswyk <abram.klooswyk@xxxxxx>]

SYHAR@xxxxxxx wrote (PHOTO-3D Digest 3158, 14 Jan 1999):
>I cannot free view
Paul Ivester answered (PHOTO-3D Digest 3159,14 Jan 1999):
> (...)[I] cannot free view anything larger than about 1.5 
>inch centers, I use a viewing aid for stereo web sites.  
>The cheapest of these is the plastic lorgnette available 
>from Reel-3D Enterprises.
> (...) I usually use an old Keystone stereoscope

Larry Berlin (PHOTO-3D Digest 3160, 15 Jan 1999):
>Apparently you have only attempted to use parallel viewing. 
>The distance of 1.5 to 2 inches is about the maximum physical 
>limit for most human beings using parallel viewing. 
>(there are notable exceptions, some on this list are
>able to parallel view 3 to 4 inch images. I can do that 
>only if the image is about 25 feet away.)

Then Larry gave an excellent description of the learning
process of the cross eyed method.

I am among the happy few which use cross-eyed as well as
parallel free viewing, the latter also with vintage Views,
which often have infinity separations over 3 inch, 80 mm
is a typical figure, some up to 90 mm. 
But this certainly gives some eyestrain ("discomfort" felt 
somewhere in or near the eyes), and I fail in parallel 
viewing when I'm tired OR when I have practiced _cross-eyed_ 
free viewing for some time :-).
But cross-eyed viewing never fails.

Larry has recommended the "finger method" as a fixation
mark, which works well mostly, but for "difficult cases"
I would recommend the "aperture method".

Make a rectangular hole of about 5 x 5 cm  (2" x 2", but size is
not critical) in a piece of thin cardboard (a cover of a glossy 
magazine will do, fold it midway, cut out 1" x 2" at the fold, unfold).
Hold the hole so that the right eye sees the left image through
it, and vice versa.

This method was recommended by Herbert McKay in his books.
(McKay, H.C., Principles of Stereoscopy, 1948 and
McKay, H.C., Three-Dimensional Photography, 1951, 2nd ed 1953.)
However, Wheatstone had presented a very similar method already
in his 1838 reading, in a paragraph _preceding_ the paragraph on
the Stereoscope.

Moreover, when the aperture method is not called an "aid for
cross-eyed free viewing", but an "aid to cross-eyed _fusion_",
then this method is even older than stereoscopy.
It goes back to a reading held in 1717, published in 1731, 
by J.T. Desaguliers. He described several fusing experiments 
with candles and other objects (this was not new), and in one 
of the experiments he used a "perforated board". There is a 
figure to it which shows only the two lateral borders of the 
"perforation", these are vertical and parallel borders, so he 
probably used a rectangular hole, or an U-shaped board.
  (J.T. Desaguliers "An Experiment to Confirm the Doctrine of 
  Refrangibility", Philosphical Transactions of the Royal 
  Society of London, Abridged edition 1710-1720, London 1731, 
  pages 181-184 and plate with fig. 64).

There is some psychophysiological evidence that such an
aperture (or maybe also a fingertip) acts as a "vergence lock",
the convergence of the eyes is more or less captured by the
fixation targets, so that it becomes easier to change 
accommodation, that is: to get the stereo images sharp.

Abram Klooswyk


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