P3D Re: prints

["Greg Wageman" <gjw@xxxxxxxxxx>]

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?


The phenomenon you are describing sounds like what is called "chromatic
aberration", and is caused by the inexpensive simple glass or plastic
lenses in the viewers you are using.  The different wavelengths (colors)
of white light are refracted by differing amounts in simple lenses (the
amount differing by the type of material as well).  This causes the
different wavelengths (colors) to come to focus in slightly different
places.  (In other words, the lens is separating the colors of white
like very much like a prism does.)  The result is the color fringing you
describe.

More expensive, multi-element lenses can compensate for this effect.
The most effective and common "achromat" design (achromat meaning
"without color") in stereoscopes seems to be  the "cemented doublet"
design, which consists of two elements cemented together, one typically
made of crown glass and the other of flint glass.  The chromatic
separation caused by the first element is reversed by the second,
resulting in virtually no chromatic aberration.

Other doublet designs may be found in stereo viewers, such as the
air-spaced doublet, consisting of a pair of plano-convex lenses
separated within a tube by approximately 1/2" or so.  These are better
in performance than a simple lens, but are not as good as the cemented
doublet achromat.  They are, however, less expensive to produce than a
high-quality cemented doublet.

     -Greg W. (gjw@xxxxxxxxxx)






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