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From: dberg@noscvax.UUCP (David I. Berg)
Newsgroups: net.rec.photo
Subject: Re: Flashes and telephoto lenses
Message-ID: <263@noscvax.UUCP>
Date: Thu, 13-Mar-86 10:25:57 EST
Article-I.D.: noscvax.263
Posted: Thu Mar 13 10:25:57 1986
Date-Received: Sat, 15-Mar-86 19:08:06 EST
References: <1587@watmath.UUCP>
Distribution: net
Organization: Naval Ocean Systems Center, San Diego
Lines: 30

In article <1587@watmath.UUCP>, rayazwinski@watmath.UUCP (Rick Yazwinski) 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.
> PLEASE HELP!!!!!

To answer the second question first, yes f4.5 is f.45 is f4.5.  The f stop
of a lens (any lens) is the ratio of its focal length to the diameter of the
lens aperture.   It measures the amount of light which will be admitted to the
flim plane.  Since the intensity of light decreases with the square of the
distance it travels, the longer the lens, the larger the diameter of the
aperture must be to achieve the same amount of light intensity at the film 
plane.  That is why a 400mm f4.5 lens has a larger diameter front element than
a 200mm f.45 lens.  (You will notice that each f stop is the square root of 
two larger than the next smaller f stop.  Eg. f1.4 is 1*sqrt2; f2 is 1.4*sqrt2;
2.8 is 2*sqrt2, etc.  This is a function of that exact principle.)

With regard to the flash question, there is light loss in a telephoto lens
(partially due to the above stated principle, and partially due to the 
abosrption by the additional lens elements).  I'm not sure that a 135 mm lens
would cause enough light loss to cost an f stop.  You might want to experiment
with this.
-- 

David I. Berg (dberg)
ARINC Research Corporation
San Diego, CA

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