P3D red button's aberrations

[Peter Abrahams <telscope@xxxxxxxxxx>]

Where there's heat, there's light.......right?  

Larry said he could
>sense the
>distortion created by the optics themselves. The entire view in a RED button
>is presented as if it's painted on the inside of a bowl. 

I believe this was an imprecise use of distortion, which should not mean
'any change in the image', but means a difference in magnification between
the center of the image and the edge of the image.  This causes a
pincushion (increased magnification at edge) or barrel (decreased mag)
appearance in lines that are near the edges.  This aberration in a red
button is very slight.

The appearance of an image as if projected into the inside of a bowl is not
something I would dismiss, though I've not seen it.  Stereo does have a way
of rendering some image qualities as plastic, sculptural, or tactile.
Aberrations are there in the image, mostly unnoticed, but the idea that
stereo can render them more plastic is very interesting.  I wonder if
anyone can see a stereo pair that is astigmatic as projected in a cylinder?
 This could be tested by making a stereoscope out of an eyedoctor's testing
spectacles, inserting two cylinder lenses, and tracking down the very few
people on earth who can detect this type of thing.

The field curvature in the score or so red buttons I've used has been
significant, far more than a modern, quality negative loupe.  This doesn't
bother many people, since our naked eye view of the world is very similar,
if not worse.  However, a flat field view, of a slide or through a
binocular, is far preferable to me.  An image through flat optics can be
projected onto a flat card and will be sharp.  Field curvature delivers an
image that would only be sharp when projected onto a curved surface, it can
be curved in or out.  To film, and to our eyes, this means that at any one
placement of film or eyes, the center of the image will be sharp, and the
edges blurred (or vice versa).

What happens in stereo viewing when the images have f.c.?  I cannot claim
to have seen a 'visualization' of field curvature, but it is very possible.
 This discussion reminded me of a letter to Sky & Telescope, May 1995, p8,
on the illusion of stereo when viewing the stars with binocular viewers
(excerpts:) 
===============
"....the intervening optics are responsible for producing the stereoscopic
effect.....the viewer really does see a three-dimensional image in most
binocular telescopes.  However, the stereoscopic cues are caused by
aberrations in the optics, particularly those in the eyepiece.....residual
field curvature and astigmatism, which present a curved image to each eye.
When the eyes are not perfectly centered over the exit pupils, the curved
images are seen from slightly different points of perspective, resulting in
a 'three-dimensional image'.  The effect is most pronounced in the least
complex eyepieces, such as Kellners and Ploessls." --R.A. Buchroeder
===============

And remember......I love my red buttons too.  However, mine don't love me back.
_______________________________________
Peter Abrahams   telscope@xxxxxxxxxx
the history of the telescope, the microscope,
    and the prism binocular


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