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From: rob@ptsfa.UUCP (Rob Bernardo)
Newsgroups: net.rec.photo
Subject: Re: Camera Vibration (really light gathering)
Message-ID: <526@ptsfa.UUCP>
Date: Sat, 23-Mar-85 10:36:24 EST
Article-I.D.: ptsfa.526
Posted: Sat Mar 23 10:36:24 1985
Date-Received: Tue, 26-Mar-85 06:59:15 EST
References: <1253@drusd.UUCP> <3468@alice.UUCP> <647@mako.UUCP>
Reply-To: rob@ptsfa.UUCP (Rob Bernardo)
Organization: Pacific Bell, San Francisco
Lines: 32
Summary: 

In article <647@mako.UUCP> jans@mako.UUCP (Jan Steinman) writes:
>
>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?

Am I missing something?
The focal length has nothing per se to do with light
gathering ability, but if we know the focal length and the ratio of focal
width to focal length, then we can determine the focal width, which does
have something to do with gathering ability.

In other words:
If the two lenses have the same f-number, i.e. ratio of focal width to focal
length, but they have different focal lengths (200mm vs 50 mm), they must have
different focal widths as well.
-- 


Rob Bernardo, Pacific Bell, San Francisco, California
{ihnp4,ucbvax,cbosgd,decwrl,amd70,fortune,zehntel}!dual!ptsfa!rob

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