P3D Chromatic Aberration (was: prints)

["Dr. George A. Themelis" <DrT-3d@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

From: Jeremy Illingworth <jeremyillingworth@xxxxxxxxx>

>When I look at my b&w prints with a replica stereo scope or the pics
>in Stereo World with the little plastic viewer, sometimes I see
>reddish yellow along edges in the picture.  What causes this and how
>can I make it go away?

What you are describing is an optical defect known as "chromatic
aberration".  This happens when the different wavelengths of light
(which correspond to different colors) do not focus at the same
spot but are spread, causing colors to appear, just like in a
rainbow or when light goes though a prism.  Simple lenses, like the
ones found in (inexpensive) stereoscopes, tend to act like prisms
for rays that are coming off the center (the further from the 
center, the worse the problem is) so this problem is inherent in 
them.  

How to get rid of these colors?

1. Use MONOCHROMATIC LIGHT to view the B&W prints.  This might be 
   the cheapest solution, but a bit impractical.  If you use light
   of mostly one wavelength (one color) then there will be no
   spreading of light in different wavelengths and these colors
   should disappear.  Some of the older Keystone viewers that I
   have owned, came with small bulbs that were colored either
   yellow or blue.  I assume that this was done for this reason,
   i.e., eliminate chromatic aberration.

2. Use ACHROMATIC STEREOSCOPES.  Best solution but rather expensive.
   Keystone made a series of achromatic stereoscopes, called "Visual
   Survey Telebinocular".  The lenses in these stereoscopes are
   achromatic, which means that they are made from two glass elements,
   one correcting the chromatic aberration caused by the other.
   Alan Lewis unearthed a small stock of achromatic lenses and
   has constructed a limited number of custom-made achromatic
   stereoscopes.  You should visit his page where he describes these 
   scopes and has a picture comparing simple vs. achromatic lenses:

   http://www.arlington.net/homes/dlewis/projects/achrom.htm

   (some detail on achromatic stereoscopes was published in our
   newsletter "Stereogram" 2.6 (Feb. '98) in the special 6-page
   section on stereo prints).

Why go ACHROMATIC?

Most good-quality slide viewers from the 50s have achromatic lenses,
but very few print stereoscopes have such lenses.  The reason is that
chromatic aberration is not as noticeable in this larger format.
But if you can see it and it bothers you (as much as it bothers me)
then you might consider investing in a good achromatic stereoscope.

In addition to eliminating these "false colors", you get noticeably
sharper images.  Chromatic aberration is a problem in color images,
not just black and white (it is just a bit more difficult to detect, 
but I see it and it bothers me).  I only use achromatic scopes (Alan
Lewis' scope and the Keystone Visual Survey) and a red button for
slides.

Sorry if this is a very long answer to a short question... and I
have nothing to sell :-)  I have sold a few Keystone scopes but
don't plan to have any for sale any time soon! (and I don't get
any commission form Alan Lewis :-) - I am just a humble devoted
fan of his...)

-- George A. (A. for Achromatic) Themelis



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