Path: utzoo!utgpu!water!watmath!clyde!burl!codas!mtune!mtx5c!mtx5d!dwj
From: dwj@mtx5d.ATT.COM (David W. James)
Newsgroups: rec.birds
Subject: Re: More on Binos: Light Gathering Ability--a Simple Formula.
Message-ID: <900@mtx5d.ATT.COM>
Date: 11 Dec 87 18:41:08 GMT
References: <9084@utzoo.UUCP> <837@cfa.cfa.harvard.EDU>
Reply-To: dwj@mtx5d.UUCP (David W. James)
Organization: AT&T Information Systems, Middletown, NJ 07748-4801.
Lines: 12

In article <837@cfa.cfa.harvard.EDU> willner@cfa.harvard.EDU (Steve Willner) writes:
>[Objective/maginification=exit pupil diameter]
Steve is right, the formula gives the exit pupil diameter.  I have
seen a formula for light gathering ability that takes into account
more than the optical efficiency and objective lens size, but have
never seen it written unambiguously, or with the justification behind it.

Another advantage of a large exit pupil is when the binoculars cannot
easily be steadied (as on a boat.)  A large exit pupil diameter
makes it easier to keep the image on your pupil.  This becomes more
difficult in bright light as your pupil shrinks.
					david james