P3D Stereo Nomenclature (part 1 of 3)

[Bruce Springsteen <bsspringsteen@xxxxxxxxx>]

Well, we converged for our American holiday of eating, and loosened our
belts to accomodate the deformation commonly known as "stretch", and then
we digest.

Those with long attention spans (there must be a few of us here) will
recall that a week ago Abram Klooswyk served an excellent six-course feast
on the subject of his favorite stereo deformation, the Third (as
enumerated by Ferwerda, who followed Bersenbrugge's list).  This was in
response to a comment from me about a month before, quoted thus:

>Bruce (making his own definitions) Springsteen wrote
>(P3D digest 3569, 28 Oct 1999): (...)
>>To me the word stretch is a very specific "deformation"
>>(Ferwerda's term for deviations from orthostereo),
>>related to angle of view (...)

My comments had been in response to a thread on 1/30 rule, close-ups, and
aesthetics wherein my two most revered stereo-oracles, George T and Abram
K, had both seemed to suggest that "stretch" was the direct and inevitable
result of stereo taken with a base too wide in proportion to the
near-object distance.  I was horrified at this apparent heresy, as I have
been on a zealous campaign this year to eradicate the misconception that
stretch is caused by enlarging base, a fallacy that is widespread and hard
to eradicate!  In an attempt to restore my shaken faith, I grasped at the
possible explanation that my friends were being uncharacteristically loose
in describing the increased parallax visible in a widened-base shot -
often taken for stretch by the uneducated eye.  I offered the following,
(in more detail):

>We seem to be talking about two things at once, and I'm confused now.  To
>me the word stretch is a very specific "deformation" (Ferwerda's term for
>deviations from ortho-stereo), related to angle of view, and unaffected
by
>high base-to-subject ratio, if I'm not mistaken.  The deformation (not
>distortion) in the portrait situation seems more due to the fact that we
>aren't accustomed to seeing shrunken heads up close (the effect we expect
>with base increase) and we find it aesthetically unnerving, as Abram
>notes. When such wide-base, near-object convergences creep too near the
>limit of our close stereo-cyclopean imaging ability, the "deformation"
>becomes a "disturbance" (Ferwerda's name for conditions that make
>stereo-viewing painful and at some point impossible). 

(...)

>BTW, this is speculative floundering on my part, not scholarship - please
>don't anyone construe this as authoritative analysis, especially you
>beginners.  I'm sure I'll be corrected in short order.

>Bruce (the Sacrificial Lamb) Springsteen

Having thus prostrated myself while grasping at possible explanation for
my mentors' seeming variance from orthodoxy, I awaited the blast of light
that was sure to follow, happy to be the sacrifice for the good of the
faith.

But it was all a mere misunderstanding!  George and Abram are ultimately
practical men, and expected me to see between the lines - to know that
when we are doing practical photography, it is most often within a viewing
system, like the Realist scheme, that will oblige us to *increase the
infinity separation* in mounting or projection, to "push" near objects
back to a set window or plane, and THAT causes the deformation known as
"convergence" or "frustum" - a clear distortion of the "space image"
proportions, and with a component similar enough to, though not identical
with, the "stretch" of the "perspective" or "focal length" deformation.  I
simply needed to know that George and Abram weren't saying "large base +
short subject distance = stretch".  They were saying "large base + short
subject distance + widened infinity spacing to push back the image =
frustum deformation which has a stretch-like component."

But I do make my own definitions, I admit, which I shall talk about next!
;-)

Bruce (the Avenging Angel) Springsteen



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