briand@tekig1.UUCP (Brian Diehm) (08/29/84)
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William Ricker questions my statement that "EV" numbers are not standard
on meters, and then goes on to make exactly my point!
Actually, his research has pointed out that the term EV is a precise
definition of a shutter speed/aperture combination. However, since EV does NOT
incorporate the film speed into the equation, no statement can be made about
the mapping of meter scales onto absolute light levels. Thus it directly
follows that:
1) Those scales on your meter are technically NOT EV scales, UNLESS
the manufacturer of the meter states the film speed for which they are
set.
2) More to the original point, the specification of the film speed
is up to the manufacturer, and is in no way defined by the equation of EV.
Now Mr. Ricker further maintains that "Any meter which reads out an EV
independent of film speed is probably giving you ASA/ISO 100 EV numbers." This
is merely another way of saying that there is a de facto standard, but that a
few manufacturers may not follow it, which is what I maintained in my posting.
For an extreme example, in 1973 John Dowdell III and Richard D. Zakia
authored the "Zone Systemizer" (Morgan & Morgan, ISBN 0-87100-040-7), a book
with a fancy dial that essentially replaced the exposure dial of your meter. It
was sort of a crutch for those learning the Zone System. One of the necessary
steps was to stick on "arbitrary numbers" onto the Systemizer dial to corre-
spond with your meter numbers - different meters used different numbers.
Mr. Zakia was at the time one of the world's foremost experts in in the
engineering discipline of sensitometry (he passed away in the late 70s), and if
he calls those number sets on the meters "arbitrary numbers", I tend to believe
him. Note specifically they were NOT called EV numbers.
Anyway, they go through two examples in the systemizer, one the Gossen
Luna-Pro, and one with the Honeywell Pentax 1/21 degree and Soligor Spot
Sensor meters. In the Luna-Pro case of the number 8, the Japanese meters read
number 3 - a difference of 16:1 in terms of light level!
In the old Basic Photo series by Ansel Adams ("The Negative", Morgan &
Morgan 1968), he points out that the number series used by the Weston Master 5
differed from the Weston Ranger 9 --- No wonder Weston went the way of the Dodo!
Anyway, these are old examples but they show the point. David Dyer-Bennet
has rediscovered this in the modern examples of the Gossen Luna-Pro F, the
Soligor Spot Sensor 2, and the Sekonic L398c. When David said in his ORIGINAL
posting that he "thought that an EV number referred to an infinite class of
equivalent f-stop and shutter speed combinations - it isn't relative to the
speed of the film", he is exactly right, and that sentence exactly explains
the possibility of his troubling observations!
When David asks how common the practice of mis-labelling scales as EV when
they aren't is, the answer must be: Any meter that labels their arbitrary
scale as EV is mis-labelling the scale! This is by definition, and is not
subject to argument - even if it is the "standard" assumed by many manufacturers
of ASA 100. The exception is if the manufacturer specifies those numbers as EV
AT A SPECIFIC FILM SPEED.
Note that all of this can be determined in meters without actually taking
any readings - this is all in the matter of what "arbitrary numbers" are
assigned to what light values. Whether two meters agree on actual light reading
is another matter entirely - one that involves accuracy of the meter, not some
assignment of arbitrary values.
As a final note to all this, in the NEW Ansel Adams Photography Series/
Book 2, "The Negative" (New York Graphic Society, Little, Brown, and Co.,
Boston, 1981), he warns of the "K Factor", from which I will quote a little:
"If pressed, the manufacturers of some exposure meters will acknowl-
edge that they depart from standard calibration of their meters by
incorporating a "K factor." This factor is supposed to give a higher
percentage of acceptable images under averate conditions than a meter
calibrated exactly to an 18 percent reflectance. The practical effect
of the K factor is that if we make a careful reading from a middle-gray
surface and expose as indicated, the result will not be exactly a middle
gray!"
This is an example of manufacturer accuracy deviation, not scale labelling
deviation. Be warned.
-Brian Diehm
Tektronix, Inc.