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Re: [OM] OMs unobtrusiveness

Subject: Re: [OM] OMs unobtrusiveness
From: Pauls0627@xxxxxxx
Date: Tue, 28 Jul 1998 11:20:34 EDT
In a message dated 7/28/98 10:48:17 AM Eastern Daylight Time,
Richard_Dale@xxxxxxxxxxxxx writes:

<< Why are fast lenses, at least below 135 mm so expensive, are they also a
 better design than the slightly slower versions, made of more expensive
 glass, that much more difficult to make or what. I can see why the giant
 telephotos cost so much, low production levels etc, but you can buy cheap
 large astronomical telescopes, presumably their lens quality is not at the
 same level. >>

In most cases, yes, the faster lenses are a better design (or at least a more
complicated one). A good example is the Zuiko 100 mm lenses. The f2 version
includes floating elements which improve image quality at close focusing

The comparison to astronomical telescopes is not a good one. You will find
most quality telescopes (by quality I'm eliminating the cheap Walmart
refractors) are slow by photographic standards with the faster ones being f4
(the popular Schmidt-Cassegrains by Celestron and Meade are typically f10).
Most of the really large aperture telescopes are of the Newtonian design, and
use only mirrors for focusing the light. Mirrors are much cheaper to
manufacture than lenses because the glass quality is relatively unimportant. I
would imagine just glass costs alone for lenses increases geometrically with
the diameter and thickness of the lens - as the size increases I would assume
the reject rate does as well, due to increased possibility of blemishes in the

Telescopes also have a different mission in life than photographic lenses.
They are typically designed to capture a large number of photons (a function
of the diameter of the objective, not the f-ratio). Stars are very nearly
perfect point sources of light, so magnification has no affect on them, just
on the image scale (how far apart two different stars appear). So a telescope
with a 10" diameter objective and a focal ratio of f/10 will image fainter
objects than a 4" diameter objective at f/4, whereas in "normal" photography
we think of the lower f-ratio being used in dimmer situations. This is
somewhat simplified, because telescopes do image things with surface area
(planets, moon, nebulae, etc.) but probably not as well as a comparable camera
lens (but who would want to pay for a 2500 mm f/8 Zuiko?)

Look at the price of a large aperture refracting telescope (4" or more).  They
are not cheap by any standards!

Sorry for the rambling.... what was the question again? <g>


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