Path: utzoo!utgpu!water!watmath!clyde!rutgers!husc6!cfa!willner
From: willner@cfa.harvard.EDU (Steve Willner)
Newsgroups: rec.birds
Subject: Re: More on Binos: Light Gathering Ability--a Simple Formula.
Message-ID: <837@cfa.cfa.harvard.EDU>
Date: 10 Dec 87 19:27:46 GMT
References: <9084@utzoo.UUCP>
Organization: Harvard-Smithsonian Ctr. for Astrophysics
Lines: 55

in article <9084@utzoo.UUCP>, snell@utzoo.UUCP ( ) says:
> 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.
All true so far, but
>            Light Gathering Ability = Objective Size / Magnification
Actually it's the square of this number times an efficiency factor to
account for the transmission of the optics.  However, the number
given by the formula is important; it's the "exit pupil diameter".
What's that, you ask?  Very simple: It's the size of the bundle of
light that emerges from the eyepiece.  That bundle has to enter the
pupil of your eye to do you any good.  Try measuring your pupil under
different lighting conditions; you probably won't find anything
greater than 3.5 millimeters or so.  (Among other things, if your
pupils are bigger, it will be too dark to read a ruler.)  Binoculars
giving bigger exit pupils are _a waste_ for (most) terrestrial
observations!  For astronomy, it's another story.  Young adults
supposedly can have pupils as large as 7mm, hence the production of
7x50 binoculars for military night use and for amateur astronomers.
(In reality, maximum eye pupil diameters of 5 or 6mm are more common,
even after long dark adaption, and children and older adults usually
cannot even achieve that.)

For birding, large lenses on binoculars are just extra weight.  You
might as well use 7x25 's, or for low-price binoculars 7x35 since the
exit pupil is often square at prices below $250 or so.  (Diagonal of
square exit pupil will be 5mm, fully filling a 3.5mm eye pupil.)  For
use in full daylight, 7x15 or 10x20 would be big enough!

>        "        "     "  3 X 30's = 10 (These are typical opera glasses)
Opera glasses are usually 3x10 or 3x12, aren't they?

>        "        "     "  7 X 80's = 11.43  (I saw such a pair in a Marine
>                                             Museum in Iceland.  They were
>                                             beautiful... and huge)
Probably 11x80, for a 7mm exit pupil.  There would be no point in
making them with less magnification.

> The Light Gathering Ability of a `zoom' lense varies depending on 
> the magnification.
Zoom lenses have greater light losses because there are more optical
elements, not to mention greater aberrations.  Use them only if you
absolutely need the zoom feature.

> 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.
Exit pupil is 4.4mm and is almost certainly round in this price
range.  The lenses are likely to be highly efficient as well, so
these have ample light gathering power for most uses.
-- 
Steve Willner            Phone 617-495-7123          Bitnet: willner@cfa2
60 Garden St.            FTS:      830-7123           UUCP:   willner@cfa
Cambridge, MA 02138 USA                     ARPA: willner@cfa.harvard.edu