P3D rules

[Peter Abrahams <telscope@xxxxxxxxxx>]

McKay's excellent writing is marred by the completely illogical statement
that in stereo photography, 'if you can't see it, it isn't there'.  I
realize I am doing him an injustice by taking it out of context, but my
issue here is the repetition of this mistake on p3d.  The re-statement of
this thinking is very counter-productive, especially for readers who are
new to mounting stereo photography & are trying to figure out just what it
is they are seeing in a stereo.

If someone is mounting their first rolls, it is _likely_ there will be
errors that will not be seen by them.  If someone is viewing with a Red
Button, there are often errors that are unseen, until the slide is
projected.  If you project a slide in a small room, there can certainly be
errors that will not be apparent, until a large screen & a large room are
used.  

How obvious are errors? There is a continuum of visibility.  Small errors
can be reduced by placing the object far behind the window.  Some errors
are 'forgivable', such as disparity caused by a small glint off a shiny
object that appears in one frame only (this enhances stereo on occasion,
but when the object is a large mirror, it is usually unacceptable).  Some
people have very 'flexible' eyes, that can accomodate errors, but others
get a headache with any viewing of misaligned images.  Small errors can
become painful in a long viewing session.  If one is using the photos to
document a scene, or for something like mapping, the tolerance for error is
greatly reduced.  And a person with long experience can often see & diagose
the smallest of errors, and tell you if it's rotation or vertical
misalignment, etc.  These tiny mistakes might be invisible, but cause a
real headache after prolonged viewing

All optical imaging systems have errors.  They are often invisible until a
superior system, with reduced aberrations, is used.  A cheap eyepiece has
distortions that are hard to see until a better one is used, and then the
astigmatism, field curvature, pincushion, etc. are apparent.  These are a
different sort of error than a mounting error, but they clarify the value
of an educated eye.

These rules are not meant for the person mounting slides to view in the
privacy of their home.  Among other things, they are designed to _help_
people who are mounting with a Red Button & want to take the slides to a
meeting with a projector.  They have certainly helped me.  I don't own a
projector & have very 'flexible' eyes, and if I tried to project my slides
at a meeting based on what I see in my viewer, it would be a terrible
experience for everyone.

To sum it up, there are often errors in slides (and prints) that are not
seen when:
--viewed by beginners
--viewed in a hand held viewer
--viewed on a screen in a small room
--the subject has no sharp edges
--the person viewing has a knack for fusing misalinged images
--viewed for a short time

If one particular person 'can't see' an error, it is _often_ there anyway.
========
PS  No one wants to reduce photography to numbers & rules.  They are part
of the reason that a successful slide works, and if they are violated, the
slide usually won't work.  They are also very interesting in their own
right, and a discussion of the math & optics is not an edict that they must
be studied.
_______________________________________
Peter Abrahams   telscope@xxxxxxxxxx
the history of the telescope, the microscope,
    and the prism binocular


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