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