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Re: [OM] Long Focus vs. Mirror Lens

Subject: Re: [OM] Long Focus vs. Mirror Lens
From: Jim Brokaw <jbrokaw@xxxxxxxxxxx>
Date: Thu, 31 Jan 2002 20:18:51 -0800
on 1/31/02 1:09 PM, Daniel J. Mitchell at DanielMitchell@xxxxxxxxxxxxx
wrote:

>> This is the case because these mirror lenses are solid catadioptric
> designs,
>> and there's no place 'inside' to put a regular diaphragm.
> 
> Ah, that makes sense. Is the Zuiko 500mm mirror not solid, and thus
> lighter? 11oz isn't all _that_ lightweight, after all. (I'd look on esif,
> but..)
> 
> -- dan

Most mirrors are 'hollow' but due to the folded light path, there isn't a
way to put in a aperture mechanism. There have been faster mirror lenses
made, but the diameter of the whole thing goes up, and there are optical
considerations in the shape of the mirrors that are affected by that. The
actual aperture (t-stop) of a mirror us usually somewhat smaller than the
f-stop due to the little secondary mirror blocking some of the front
opening. I have a truly solid catadioptric lens, made by Vivitar, and its
extremely small for its focal length, but being a solid lump of glass it is
very heavy for its size. The one advantage of the truly solid design is that
the mirrors, being the ends of one lump of glass, stay in alignment. The
alignment of mirrors in a mirror lens is critical, and this is one area
where low cost mirrors can fall down some... the 500/8.0 Zuiko is much more
solid that the $125 mirror lenses I've ever seen. It is not more solid than
my 600/8.0 Vivitar Solid Cat though, but it is about 1/3 as heavy... I also
have a 300/5.6 mirror (says Celestron but I bet the others are all made by
the same factory) that is smaller in diameter and length than my 100/2.0
Zuiko, and much much smaller than the 300/4.5 lens. The tradeoff is the
fixed aperture and the speed, but for what it does it works well. The cost
is about 1/3 the Zuiko also, if you don't use a 300 or 500 much that is a
consideration.
-- 

Jim Brokaw
OM-1's, -2's, -4's, (no -3's yet) and no OM-oney... 


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