[LargeFormat] ASA jump... when?

[Richard Knoppow]

----- Original Message ----- 
From: "LNPhoto" <LNPhoto@twmi.rr.com>
To: "f32" <largeformat@f32.net>
Sent: Friday, February 27, 2004 1:53 PM
Subject: [LargeFormat] ASA jump... when?


> The fact that the the ASA film speed jumped sometime back
in the 50s
> has been discussed here before, what I need to know is
when it happened
> so that I know brochures, equipment, etc before X date
used the old
> system and after used the new.
>
> Les
>
  I can't give you the exact date right now but it was about
1958. The ASA changed the method of measuring film speed
from a variation of the Kodak method to the DIN method. The
two methods in fact come up with about the same speed for
most film, the difference in published speed was due to a
very large safety factor, 2.5 times, included in the
original ASA method. This was cut in half to a 1.25 X safty
factor, which remains today. As a result all film speeds
about doubled.
  The Kodak method originally was intended to give the
minimum exposure which would result in good shadow detail.
In order to do this the contast of the toe area had to
measured separately from the average contrast. The exposure
point was where the contrast of the toe was 1/3rd that of
the average contrast. This is a difficult measurement to
make. The ASA adopted the DIN method, still used today,
because it is based on a fixed minimum density above gross
fog and base density. This is much easier to measure.
According to the ASA at the time many films were measured by
both methods and not enough difference was found to justify
the additional difficulty of the Kodak method. The safety
factor was removed because it resulted in overly dense
negatives. In fact, it went against the whole idea of the
Kodak speed method as developed by L.A.Jones of Kodak Labs.
  Up until the adoption of the DIN method by the ASA there
were a number of "magic" developers on the market promising
to yield up to two stops of speed improvement. Since the
film was already rated one stop too slow, and since there is
still a stop of latitide even at the DIN speed, they could
deliver. When the new ratings came out all of these
developers disappeared from the market in short order.
  I can probably find the day and year the new ASA standard
was adopted but it will require searching through a couple
of books.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com