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Path: utzoo!snell
From: snell@utzoo.UUCP ( )
Newsgroups: rec.birds
Subject: More on Binos: Light Gathering Ability--a Simple Formula. 
Message-ID: <9084@utzoo.UUCP>
Date: Wed, 9-Dec-87 17:37:58 EST
Article-I.D.: utzoo.9084
Posted: Wed Dec  9 17:37:58 1987
Date-Received: Wed, 9-Dec-87 17:37:58 EST
Organization: U of Toronto Zoology
Lines: 36


Should have mentioned this in previous posting. (Also, no, I don't
own rubberized binoculars.  I think they are foolish and unnecessary
for the reasons I stated).

Lovely Light.  The more your binos take in, the better.  The trade off
is that good light gathering plus magnification equals weight.
A simple formula will help you easily determine how good a set of binoculars
will be at this essential task.  Weight and optical quality are separate
considerations.

           Light Gathering Ability = Objective Size / Magnification

Thus Light Gathering of 12 X 30's = 2.5  (I've seen such at K-Mart).
       "        "     " 10 X 30's = 3  (I've seen these at a K-Mart too).
       "        "     "  9 X 40's = 4.44
       "        "     "  7 X 35's = 5
       "        "     "  8 X 40's = 5
       "        "     " 10 X 50's = 5
       "        "     "  7 X 50's = 7.14
       "        "     "  3 X 30's = 10 (These are typical opera glasses)
       "        "     "  7 X 80's = 11.43  (I saw such a pair in a Marine
                                            Museum in Iceland.  They were
                                            beautiful... and huge)

The Light Gathering Ability of a `zoom' lense varies depending on 
the magnification.

Obviously Leitz 9 X 40's do not come out as winners in the Light Gathering
War, but the optics are so much superior that they more than compensate.
__
Name:   Richard Snell
Mail:   Dept. Zoology, Univ. Toronto
        Toronto, Ontario, Canada    M5S 1A1
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