P3D The Third Deformation of 3D, Seeing or not seeing Her (5 of 6)

[abram klooswyk <abram.klooswyk@xxxxxx>]

I have tried to discuss the origin of the Third Deformation,
but lets look at cases where she is hiding or exposing
herself, and why.

In superimposing infinity points on the projection screen at
7 feet , causing 2 degrees of viewing convergence to infinity
homologues, do mountains shrink to mole hills at 7 feet?
I don't think so.
Only when an even larger "crossed disparity" is used, with
right hand far point homologues to the left of the left hand
ones, the shrinking will occur, a puppet theatre effect can
be seen. I believe that at least 3 to 4 degrees of eye
convergence is necessary to see it.

Moving near points out on the screen, so that near points are
seen with parallel eye axes, does this move near objects to
infinity, and enlarge them correspondingly?
No, certainly not to infinity and hardly enlarged.
Two degrees of divergence in seeing infinity objects also
doesn't seem to give a perceptual effect.
This shows the failure of math-only theories of stereo
geometry.

But is this third deformation seen at all in normal practise?
I have mentioned the close-up shots with Realist type cameras.
Many similar close-ups with a (too?) large base can show the
same effect. When these slides *also* have a too great depth
range, the far points become also infusible far spaced on the
screen, a combination which Ferwerda called "close-up misery".

In other cases of the third deformation there is mostly a
combination with one or two of the other deformations.

I have written on the Macro Realist slides, which often show
marked stretch, especially with recognizable familiar objects.
Flowers of which you know they are perfectly circular can
become oval, stretched in depth.
This are macros, so they can show some gigantism (First
deformation). Under the right viewing circumstances they
don't suffer from stretch due to the Second or distance
deformation. But they do show the Third one.
The base is 15 mm for a distance of 10 cm, which is over 8
degrees of photographic convergence. Viewing this with only
2 degrees of convergence is a too large difference to be
ignored by the visual system.

A complex case is Pepax. A 19th century theory says that
the squeeze caused by long camera lens focal distances and
normal viewing focal length (2d or distance deformation)
can be compensated by a larger base (1th deformation).
If camera focal length is "n" times viewer focal length,
you could compensate the squeeze by increasing the base
also "n" times, the theory says.
I have argued before (last time in P3D 2917, 22 Aug 1998,
"PePax principle is incomplete") that the depth squeeze
deformation by the long lenses (2d deformation) affects only
one of the three dimensions: depth, so it cannot be corrected
by the base deformation only, because the hyperstereo or
lilliputism effect gives *equal* diminishing of *all* three
dimensions.
But adding the third deformation, by enlarging the base even
more, so that photographic convergence is increased too,
will do the trick. (The theoretically also occurring frustum
effect seems not perceptible, at least for me it isn't.)

This sounds very complex, but in practise no calculation is
needed, a roll of tests shots will do. Many stereographers
use this technique after only some experiments, without any
calculation. For small objects, I like to call the technique
"telestereo close-up". Many great pictures have been made
with it, I remember most vividly shots by Paul Wing, Rudi
Wysnevski and especially action shots by Allan Griffin.
(To be continued)

Abram Klooswyk