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re: [OM] 500mm relfex lens question

Subject: re: [OM] 500mm relfex lens question
From: "dlanor" <dlanor@xxxxxxxxxxxxxxx>
Date: Mon, 19 Jun 2000 19:20:14 +0800
: WJDuprey@xxxxxx  William asked:  beginner question.  What is reflex mirror
lens? And what is the term Catatrioptic mean?  Thanks for the information.

William,
to easily summarise, a reflex lens is a way of making a lighter, shorter
lens than otherwise would be needed.  Generally 500mm (1/2meter) lens would
be that long, that is 500mm "focal length" to the film plane.  In longer
lenses element design is used to reduce the "actual length" and achieve the
same result.

In a reflex lens, mirrors are used to reduce the length.  The front is clear
except for a blocking circle in the middle.  Light enters, goes to a mirror
at the back of the lens, this reflects it forwards to the lens front, where
it reflects off a second much smaller mirror (the other side of the blocking
circle).  The light then returns through the lens to the rear again and
enters the camera through a hole in the middle of the first mirror.  This
bending and use of mirrors gives 500mm focal length.  Basically a hollow
tube with a big mirror and a small mirror, saving weight as mirrors are
lighter than lenses.  A rough diagram (use imagination,  hope it lines up
properly) in cross section:-
.
            --------------------------   lens barrel
-light >>G>>>>>>>>>M I
             Im<<<<<<<<<M I----
             Im>>>>>>>>>>>>>>>>> to CAMERA    G=glass  M=mirror
             Im<<<<<<<<<M I----
-light >>G>>>>>>>>>M I
            --------------------------   lens barrel

Otherwise a 500mm lens would be much heavier and longer.
Hope that helps
Ron Ligtermoet
dlanor@xxxxxxxxxxxxxxx


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