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Re: [OM] Lenses

Subject: Re: [OM] Lenses
From: "John Petrush" <petrush@xxxxxxxxxxxxx>
Date: Fri, 27 Aug 1999 23:23:57 -0400
There are several different designs of "mirror" lenses, but they all share
similar characteristics.  They are sometimes called catadioptric or
cassigrain lenses.  They employ two mirrors to "fold" the light path,
yielding a long focal length in a compact package.  As the light enters the
front of the lens, it passes through a corrector plate, then the primary
(parabolic or spherical) mirror at the base "nearest the camera body" of the
lens.  It is reflected off a first surface mirror and concertrated on a
second, smaller mirror on the *back* side of the corrector plate, where is
is reflected again through a hole in the middle of the primary mirror and
focused on the film plane.

The advantages are primarily a small, light weight package for long (>350
mm) lenses and virtually no chromatic abberations (color seperations).  An
all-glass design must solve the problems encountered by the fact that light
of different wavelengths focus at different points.  Lenses that
successfully do this are often called apochromatic (APO) lenses.  Read
dreadfully expensive <g>.  The trade-offs include a fixed aperture and low
contrast caused by the central obstruction.  They are generally slow focal
ratios (f/8 and slower - there are a few faster versions).

This same basic optical design is used for astronomical telescopes and
popular "spotting scopes" used by bird watchers.  Build quality varies
greatly.  The Zuiko 500mm f/8 mirror telephoto is a fine example of
excellence in design and build.

John P
______________________________________
there is no "never" - just long periods of "not yet".
there is no "always" - just long periods of "so far".


Jenna <Eden@xxxxxxxxxxxxxxxxxxxxxx> wondered:

> Could some kind person explain what 'mirror' lenses do?




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