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Path: utzoo!watmath!clyde!bonnie!akgua!mcnc!decvax!cca!ima!ISM780!ed
From: ed@ISM780.UUCP
Newsgroups: net.rec.photo
Subject: Re: exposure stop formulae
Message-ID: <81@ISM780.UUCP>
Date: Sat, 2-Feb-85 01:21:21 EST
Article-I.D.: ISM780.81
Posted: Sat Feb  2 01:21:21 1985
Date-Received: Mon, 4-Feb-85 05:26:03 EST
Lines: 30
Nf-ID: #R:ttidcb:-25400:ISM780:21200011:000:1013
Nf-From: ISM780!ed    Feb  1 09:38:00 1985


>     I've been meaning to post this for a while and keep
>forgetting...for the mathematically inclined, the following formulae
>can be used for calculating the ubiquitous difference in stops between
>two f stops or two guide number (first formula) or between two shutter
>speeds or two ASA/ISO ratings:
>
>                   #stops = 2*(log(fg1/fg2))/log(2)
>
>                     #stops = log(sa1/sa2)/log(2)
>where:
>     fg1 and fg2 are two f stops or two guide numbers,
>and  sa1 and sa2 are two shutter speeds or ASA/ISO ratings.
>
>                        Bill Josephs
>                        TTI
>                        Santa Monica, Ca.
>/* End of text from ISM780:net.rec.photo */

You didn't mention what log base to use, although upon closer inspection
I see it doesn't matter.  In that case, for most situations, base 2
will be easiest to calculate (sans calculator), so these simplify to:

	#stops = 2*log (fg1/fg2)
		      2
	#stops = log (sa1/sa2)
		    2
Ed Lycklama
decvax!cca!ima!ism780!ed