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Path: utzoo!watmath!clyde!floyd!harpo!decvax!decwrl!dec-rhea!dec-nermal!hagerman
From: hagerman@nermal.DEC
Newsgroups: net.rec.photo
Subject: lenses
Message-ID: <1545@decwrl.UUCP>
Date: Fri, 15-Jun-84 11:13:58 EDT
Article-I.D.: decwrl.1545
Posted: Fri Jun 15 11:13:58 1984
Date-Received: Thu, 21-Jun-84 03:13:06 EDT
Organization: DEC Engineering Network
Lines: 27

x
re:  lenses sharpest at medium f/ stops.

If you don't think it's possible to build a lens that is sharpest
at its widest opening, consider this:

My lens is sharpest at, say, f/8.  I put a mechanical stop on the
aperture (note spelling) control so that it can only move between
f/8 and, say, f/16.  Now I have a lens that is sharpest at its
widest opening.

If this sounds far-fetched, try measuring the diameter of the front
element of some of your lenses and then calculating what the
maximum f/ stop should be.  You'll find, in many cases, that the
front element is larger that you would expect.  (Specific example:
Leitz Summitar 50mm f/2.0 works out to about f/1.5.)
This is to minimize vignetting problems.
The manufacturer can choose between making a lens that is faster
but not quite as sharp or a slower lens that is sharper.  In fact,
Leitz claims that their 50mm f/1.0 is as good as their 50mm f/2.0
lens.  The problem is that the f/1.0 lens is a monster.

The point is that lens design, like a lot of other things, involves
compromises, and the result is that most lenses are sharpest
at medium f/ stops.

D. Hagerman