[photo-3d] Achromatic vs. Single-element lenses

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

At 09:31 PM 9/17/00 EDT, you wrote:
>Pardon my ignorance (yet again), but what is "achromatic" and 
>how does it compare with non achromatic lens?

We are talking about viewing lenses here... A simple
lens (just one piece of glass) suffers from chromatic
aberration.  Different parts of the light spectrum
(say blue vs. red) will be focused at different
planes.  As a result the image will appear to have
color fringes (red and blue).  You can see that if
you carefully examine a black and white image in a
viewer that has single-element lenses.  Any colors
that you see come from the lens....  In addition,
a single lens suffers from other aberrations that
reduce sharpness off-center, like spherical aberration.

As Kingslake puts it: "it was early discovered that
a weak negative lens of a highly dispersive material,
such as flint glass, could be combined with a
stronger positive lens of a less dispersive material,
such as crown glass, to form an achromatic lens having
no chromatic aberration."

So the achromatic lens consists of two elements, two
pieces of glass, cemented together.  It shows no chromatic 
aberration.  In addition achromatic lenses are designed
so spherical aberration is also reduced.

Achromatic lenses cost considerably more than single
element lenses so they are found in the more expensive
stereo viewers.  They make a very fine viewing lens.

George Themelis