From: Richard Koolish (koolish@alum.mit.edu)
Date: 12/18/02-02:57:26 PM Z
re: telephoto lens design ...
A tele-converter for a 35 mm lens shows how telephoto lenses work.
It is just a negative lens inserted in the optical path between the
original objective lens and the film plane. If you imagine a couple of
light rays from the original lens converging toward the film plane,
the negative lens causes them to converge more slowly, thus acting
as if they came from a longer lens. A Barlow lens for a telescope
does the same thing, increasing the magnification of the original
eyepiece. Telephoto lenses have the teleconverter built in instead
of adding it on later.
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