wdr@security.UUCP (William D. Ricker) (08/01/84)
From: ddb@mrvax.DEC (DAVID DYER-BENNET MRO1-2/L14 DTN 231-4076)
Subject: EV
Message-ID: <2729@decwrl.UUCP>
Date: Fri, 20-Jul-84 11:10:28 EDT
.
:
The Gossen Luna Pro F indicated essentially the same
exposure, and called it EV 8. The Soligor Spot Sensor 2, which
reads directly in EV, said EV 6! Quick playing with the
calculator dial produced the same actual exposure, however.
Now, I've always thought that EV, meaning exposure value, was a
standardized system. It's used by many manufacturers to
indicate the max and min sensitivities of meters, for
example ("at asa 100, from EV -1 to EV 18"). Furthermore, I
thought that an EV number referred to an infinite class of
equivalent f-stop and sutter speed combinations. It isn't
relative to the speed of the film.
Davy was wondering what the heck was going on, so he
called his friend Stevie and asked him to check what EV 8
meant to his Sekonic L398c studio meter. It turned out to
agree with the Luna Pro F; and both of them agreed that film
speed had nothing to do with it.
The bloody Soligor Spot Sensor II continued to disagree,
however. The meanings of its EV numbers were
film-speed-sensitive. They agreed with the other two meters
only at ASA 100.
As far as I can tell, after looking through my book collection
(which wasn't at all helpful), and paging through the books
at a local bookstore, what's going on is that the Soligor Spot
Sensor II has a scale that ISN'T EV which it labels as EV!
Can anybody comfirm or deny this diagnosis? How common is
the practice of mis-labeling scales as EV when they aren't?
-- David Dyer-Bennet UUCP:
...!{allegra|decvax|ihnp4|purdue|shasta|utcsrgv}!
decwrl!rhea!mrvax!ddb
I do not wish to contradict Brian Diehm, who carefully recommends
cross-calibrating before comparing EVs -- in light of the evidence,
this is good advise.
But.
My Kodak Pocket Guide Gives EV on its Multifunction dial as a function
of Aperture and Shutter speed; (or, by transitivity, of filmspeed and
illumination). The mathematical relation modeled by the dial is
log2(shutter-duration) + 2log2(fstop number) = EV
Purists may read this as
EV = -log2(shutter in seconds) + -2xlog2(aperture in focal lengths)
I have an ancient little silicon cell meter, which clips in the shoe.
It reads only EV given film speed has been empirically demonstrated to
use this definition of EV. I have found this formula handy for setting
the camera without looking up tables. (As a computer scientist !?! I
am more at home with Log2 than Log10 or ln anyway.)
Kodak's definition may not be standard, but it is elegant and useful.
Note that the magazines and Ads always say EV AT ASA 100 (or ISO 100).
Given the illumination, the EV is dependant on the film speed, or on a
standard film speed. Any meter which reads out an EV independent of filmspeed
is probably giving you Asa/Iso 100 EVs. If it doesn't, then its really weird.
Note: DIN film speeds(the other half of ISO nomenclature) are 3log2(ASA)+C
for some constant C. Thus a DIN is a third-stop. This corresponds to the
ASA ratings of 25,32,40,50,64,80,100,128,160,200,256,320,400, etc.,
which are rounded in commerce. The DIN scale has the nice property that
adding 10 to the DIN is the equivalent of multiplying the ASA by 100.
(as 2^10 ~~ 10^3, 2^10/3~~10) Somewhere in one of the recent magazines
there is an article relating foot-candles to ASA & exposure....
Happy shooting..
--
William Ricker
wdr@mitre-bedford.ARPA (MIL)
wdr@security.UUCP (UUCP)
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Opinions are my own and not necessarily anyone elses. Likewise the "facts".