From: Richard Knoppow (dickburk@ix.netcom.com)
Date: 02/11/00-05:48:49 PM Z
At 12:34 PM 02/11/2000 -0800, you wrote:
>Hi!
>I was wondering if anyone knows just when we began to
>use ASA numbers? I was looking at 1939-1950 magazine
>ads for Ansco and Kodak sensitive materials and the
>the lack of speed information is, well, noticeable.
>
>Any one know of a chart listing the equivalent speeds
>for the former H&D speeds with the ISO (or ASA)
>speeds?
>
>Thanks... and sorry about the off topic(?)subject, but
>the recent inquires of that um... commercial entity
>just seemed to open this can of worms...that stuff
>about international standards of light and...well if
>you have any information, use your own discretion as
>to whether it should go on or off list. Thanks.
>
>Ray
The dates here are from memory and approximate.
ASA speeds were adopted in 1946. They are based on a method of speed
rating developed at Kodak labs by Lloyd A. Jones in a long research project
on tonal reproduction. Jones results were published in detail, probably the
most complete summary is in _Theory of the Photographic Process_ revised
edition, C.E.K. Mees. The speed measuring method was based on finding the
minimum exposure which would result in an "excellent" print as judged by a
very large panel of both expert and naive judges. It was a double-blind
test of thousands of photographs.
The method developed specified a minimum gradient of the toe portion of
the curve as well as a point on the straight line portion. This resulted in
a speed measurement which was relatively un-affected by the curve shape of
the film, not true of previous methods.
Kodak introduced this method for their products around 1940. The first
ASA speeds were obtained using Jones' method but added a 2.5X safety
factor. This resulted in quite dense negatives. In fact, Kodak advised
using higher speeds if the negatives were too dense. Since Jones' work was
to find the minimum exposure for good shadow detail the ASA speeds undid
much of his work.
The ASA method was revised in 1960. The method adapted was the then
current DIN method, which used a fixed point of minimum density and
specified the gamma and maximum density to be gotten from a certain range
of exposure. It is essentially the same method used now. While thsi method
is not so immune from differences in curve shape as the minimum gradient
method the results are much the same and its much easier to measure
reliably. At the same time this method was adopted by the ASA it also
dropped the safety factor, effectively doubling the speeds of all B&W
negative films (color and reversal films are rated by a different method).
The current ISO method is essentially the same as the 1961 ASA method
except that specifications for a specific developer were dropped. Any
developer can be used now, provided it is specified along with the
resultant speeds.
Earlier methods of speed measurement are not directly convertible into
ASA or ISO speeds. However, conversion factors for light meter calculators
are practical.
There are conversion charts in older handbooks which give some idea of
the comparative speeds of ASA, DIN, Schneiner H&D, Weston, etc. speeds.
There were at least two H&D speeds and at least two Scheiner speeds, so one
must be careful when using these charts.
It would appear that the general purpose films available in the mid
thirties were around ISO 30, fast films around ISO 80. In 1935 Agfa
researchers found out how to sensitize film with gold and just about
doubled the speed of their films without grain increase. It took Kodak a
couple of years to catch up, but there was a rather sudden increase in film
speeds and reduction of grain around the late 1940's.
While photolytic exposure meters (Wynne, Watkins, etc.) were available
from the 'teens the first photo-electric exposure meters didn't appear
until the mid 1930's. The Weston meter was the first to become popular,
partly because they devised a reliable speed measureing method and did the
rating themselves. Previous methods, particularly the Scheiner method,
could be easily fudged by the manufacturer to get higher speed numbers.
The practice in the motion picture industry throughout the 1930's and
40's was to make actual sensitometric tests on each batch of film to
determine its speed. Exposure was largely from experience with standardized
lighting techniques. The industry started using photoelectric metering when
the Weston meter became available meters were not popular until after 1945.
Specifically, conversion charts for various speed methods can be found in
old British Journal Annuals. ISO speeds can be used directly in meters
calibrated for ASA speeds. For Weston meters use the next lower speed, for
old General Electric meters, use the next higher speed.
My old Weston Master II reads such that I suspect it corrected for a
safety factor in the original Weston speeds. i.e it gives about one stop
less exposure than a current meter does. A defect in the selenium cell
would result in indicating a lower light level and greater exposure. Most
used GE meters are already set up for ASA speeds rather than the older GE
system.
---- Richard Knoppow Los Angeles,Ca. dickburk@ix.netcom.com
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