[photo-3d] Chromatic aberration

["Dr. George A. Themelis" <drt-3d@xxxxxxx>]

Monte Ramstad wrote:

> As the inventor of a prismatic stereoscope, I would like to
> give my two cents worth about chromatic aberration.  But
> before I do, I'm wondering if a few people would be willing
> to send me some digital images which illustrate the problem
> of chromatic aberration.

Chromatic aberration is something that you see when
you look through a single-element simple lens.  One way
to show it is to take a picture of this image through a
digital camera and I will try to do this tonight.

All stereo slide viewers with single-element lenses show
chromatic aberration.  One of the most notorious IMO
is the View-Master model D.  When I got my model D
from  Reel 3-D in 1989, I was socked by the amount
of bluish tint at the view edges and returned the viewer
promptly.  You see, I was used to the fabulous red
button with the good achromatic lenses.

In simple terms, light is not monochromatic but it is made
of different wavelengths (each with its own color).  The
refractive index is different of each wavelength and, as
a result, different wavelengths of light will be focused
at different lenses by a simple lens.  The result is color
fringes (blue or orange) around the image.  The best
way to see it is to look at a black & white image around
the edge of field of view.  Heck, you don't even need
to look at a slide, just look at the apertures of the viewer.

One way to eliminate chromatic aberration is to use
monochromatic light in the viewer.  This of course
does not work for color images but it could work
with black and white images.  My guess is that this
was tried by Keystone because some of the light
attachments for their Telebinoculars use blue bulbs.
Things are better but you can still see some chromatic
aberration because this light is not very monochromatic
(i.e. one wavelength).

The other solution is to use two lenses.  The second
element corrects the chromatic aberration of the first.
This is done for 2 or 3 wavelengths of light and this
is the principle of the achromatic lenses.

Now, it is clear that if you use mirrors or direct
vision, you don't get any chromatic aberration.  You
must use lenses or prisms.

George Themelis