[LargeFormat] ASA jump... when?

[LNPhoto]

Interesting, I had pegged it about 1954.
On Friday, February 27, 2004, at 09:21  PM, Richard Knoppow wrote:

>
> ----- 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
>
>
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