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From: tsp@hadron.UUCP (T. Scott Pyne)
Newsgroups: net.rec.photo
Subject: Re: Flashes and telephoto lenses
Message-ID: <297@hadron.UUCP>
Date: Wed, 12-Mar-86 22:15:57 EST
Article-I.D.: hadron.297
Posted: Wed Mar 12 22:15:57 1986
Date-Received: Sun, 16-Mar-86 00:45:30 EST
References: <1587@watmath.UUCP>
Reply-To: tsp@hadron.UUCP (T. Scott Pyne)
Distribution: net
Organization: Hadron, Inc., Fairfax, VA
Lines: 15

[]
In article <1587@watmath.UUCP> rayazwinski@watmath.UUCP writes:
>I have heard that when using a flash and, say a 135mm lens you have to change
>you f-stop to allow for the light loss through the extra glass.
>I have also heard that f4.5 is f4.5 no matter which lens you use.

The latter is true.  f4.5 is actually f/4.5, meaning that the
aperture = the focal length / 4.5.  Thus, the actual aperture
is larger with a 135mm lens than with a 50mm lens for a given
f/ stop.  If the calculated f/ stop for a given flash-subject
distance is f/4.5  then it will be f/4.5 for any focal length
lens,  modulo certain situations involving extension tubes or
bellows.
					Scott Pyne
					...!seismo!hadron!tsp