P3D David Lee's Contributions and Other Stuff (Part 2)

[Bruce Springsteen <bsspringsteen@xxxxxxxxx>]

This is a continuation of a train of thought in the new APEC maillist,
but I will open a "second front" by cc-ing P3D, as the topic is
related to a recent thread there as well.  I apologize for
double-dipping your inboxes. Think of it as cross-pollination.

Stereo base choice is an evergreen stereo topic, as evidenced on both
lists lately.  But I sense some persistent confusion over the basics -
some of which may be mine. ;-)  May I discuss an example?
 
David Lee recently sent a "hyper" stereocard to APEC members,
depicting the western American marvel "Devil's Tower".  It is a black
& white pair, taken on infra-red film in his trademark style.  It's a
dandy.  It also is a good example of the effect of very large-base
scenics, I think.  In the foreground is a rolling expanse of land
(perhaps a river valley?) covered with trees.  Standing in the
far-middle distance is Devil's Tower, a unique geological object
rising 867 feet from its base - looking like the crown of a sombrero
standing alone.  Behind are dramatic clouds.  

In his notes David says the picture was taken with two cameras 20 feet
apart.  Whether free-viewed or in a viewer, the stereo clearly shows
the effects of increasing the base far beyond normal human eyespacing
- that is the subject appears smaller and closer that it would in
reality.  Since 20 feet is 96 time the average interpupillary distance
of 2.5 inches, that means that the tower appears 1/96th its actual
size and 1/96th its actual distance.  This is often called
"Lilliputism" - the scene, trees and all, looks quite like a scale
model.  I'll come back to this.

To change stereo base is to change "parallax" relationships - the
angle between the two eyes when converging on the various parts of the
scene.  Small close things show more parallax differences than do
large distant things.  So increasing base adds to the sense of "depth"
by shrinking the whole scene into the range where parallax differences
are most pronounced. What does not happen is any distortion of the
SHAPES of things - they are not stretched or squashed at all - a
common misconception.  THAT is a function of "perspective" ie: the
relationship between "taking" and "viewing" focal lengths, and the
resulting angle of view that the picture fills in your field of
vision.  In David's card, the miniature appearance of his trees,
clouds, and tower is a given - no matter how I view that card -
because of his 96 times normal stereo base.  But I could stretch or
squash the depth dimension by changing the WAY I look at the picture -
holding the card nearer or farther when I freeview, or using viewers
with different magnifications (focal lengths).  David has no control
over that. All he can do is tell me the focal length of his camera
lenses, and the amount the prints have been enlarged, and hope that I
will look at his card in a way that doesn't deviate TOO much from the
correct perspective.

So back to base, which is my real subject.  Pronounced increases in
base shrink the subject ("Lilliputism"), pronounced reductions in base
enlarge the subject ("gigantism" or perhaps "Brobdingnagism" to follow
the Swiftian metaphor?).  So making choices about base should be
thought of as making the world look larger or smaller, and thereby
changing your point of view in a way not possible by other means. 
Which suggests another way of thinking about this - the one I prefer,
which I call "Gulliverism".

When I look at David's Devil's Tower, I make a mental shift and
imagine myself 96 times my current size - the height of a 58 story
building - lying on my belly looking at the scene before me. 
Alternatively, if I were to take a "hypo" or reduced base shot looking
over my kitchen sink, I would shape it in my mind as the view a tiny
man might have looking into a great chasm - a Grand Porcelain Canyon
scenic with towering forks and water glasses in the middle distance
and bread crumb boulders around me.  And I would pick a stereo base
that matched the eye spacing that such a tiny man might have.  In a
recent SSA folio, I saw a sequential hyper taken from a helicopter
over the ocean, looking in at a Hawaiian beach with lush green
mountains behind.  Rather than thinking of a miniature Hawaii, I
imagined I was Godzilla, wading in for some fun.  

And nothing looked distorted.  Just yummy.

Bruce.

         
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