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From: jans@mako.UUCP (Jan Steinman)
Newsgroups: net.rec.photo
Subject: Re: Camera Vibration (really light gathering)
Message-ID: <647@mako.UUCP>
Date: Sun, 17-Mar-85 15:27:24 EST
Article-I.D.: mako.647
Posted: Sun Mar 17 15:27:24 1985
Date-Received: Thu, 21-Mar-85 02:19:12 EST
References: <1253@drusd.UUCP> <3468@alice.UUCP>
Reply-To: jans@mako.UUCP (Jan Steinman)
Organization: Tektronix, Wilsonville OR
Lines: 26
Summary: 

In article <3468@alice.UUCP> ark@alice.UUCP (Andrew Koenig) writes:
>Phil Lavette says:
>
>> Similarly, there is a two-fold increase in brilliance achieved when a 60mm
>> main lens is substituted for one 50mm in diameter...
>
>I do not see how I can parse this as a true statement.  The obvious meaning
> -- that a 60mm lens puts twice as much light on the film as a 50mm lens --
>is false.  How much light reaches the film depends on only two things: the
>amount of light on the subject and the f-number of the lens.  Focal length
>has nothing to do with it.

1) Phil specifically mentions "diameter", or focal width.  It should be
obvious that a wider lens gathers more light.

2) Less obvious is that focal length has as much to do with light gathering
ability as focal width.  The "f-number of the lens" (ignoring losses from
difraction and internal reflection) is actually the ratio of focal width to
focal length.  If "focal length has nothing to do with it", why does a 200mm
f4 lens have a larger front element than a 50mm f4 lens?

Without defending Phil too much (I don't have the original posting), you are
wrong, Andrew.
-- 
:::::: Jan Steinman		Box 1000, MS 61-161	(w)503/685-2843 ::::::
:::::: tektronix!tekecs!jans	Wilsonville, OR 97070	(h)503/657-7703 ::::::