Re: [OM] Lenses

Subject: Re: [OM] Lenses
From: Acer Victoria <siddim01@xxxxxxxxxxxxxxx>
Date: Fri, 27 Aug 1999 16:21:49 -0700 (PDT)
On Fri, 27 Aug 1999, Eden wrote:

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

It's just another lens, doing the same job all lens do: focussing light
onto the film plane. However, a mirror lens does it differently from
'regular' lens.
Generally speaking, the physical length of lens is almost the same focal
length. Now, if you wanted a 500mm lens, it would be a monstrous long
beast, no to mention heavy because of all the glass in it. Mirror lenses
reflect the light inside the barrel back and forth once, in effect
increasing the path the light travels and thus the focal length. I'm
afraid that wasn't a very good definition, so let's see how well I can
draw ;-)

"Normal" lens, say 500mm
--->                            |                          |
--->light rays from source--->  |                       ---->film       
--->                            |__________________________|

"mirror" lens, 500mm
--->                            |               |
--->                            |               |(m1)
--->light rays from source      O  |(m2)--------------->to film
--->                            |               |(m1)
--->                            |               |

The light goes in, bounces off the mirrors labeled (m1) and onto m2, and
then from m2 it bounces to the film. Make a litte more sense? That's why
only longer lenses come in mirror form, 500mm being the most common it
seems. The above numbers are made-up to demonstrate only, but the idea is
the same.
With mirror lenses, you don't a choice of setting aperture, or the size of
the lens opening; it's fixed, say at f:8. They are cheaper and lighter
than refracting (normal) lenses.
Hopefully I didn't butcher up some concept--if I did, you'll hear about it
very shortly when I get corrected by the 'elders' <G>

/Acer "steppenwolf" Victoria

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