[pure-silver] Re: Agfa Paper Equivalent
- From: Bogdan Karasek <bkarasek@xxxxxxxxxxxx>
- To: pure-silver@xxxxxxxxxxxxx
- Date: Mon, 09 Jun 2008 23:48:03 -0400
WOW!
Thank you, Richard, that was a great little jewel of photo history!
So Tri-X jumped from 200asa to 400asa, or was it around then?
On that same item, when did XX morph into Tri-X?
Cheers,
Bogdan
Richard Knoppow wrote:
----- Original Message ----- From: "john stockdale" <j.sto@xxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Monday, June 09, 2008 3:55 PM
Subject: [pure-silver] Re: Agfa Paper Equivalent
At 08:17 AM 10/06/2008, Richard wrote:
.
At one time AGFA contrast numbers were one number
higher than Kodak for the same contrast, i.e., AGFA No.3
was the same as Kodak No.2 but AGFA changed this many
years ago (1980s I think but am not certain).
---
Richard Knoppow
Maybe that has something to do with their film development
instructions leading to eccessive contrast. I'm sure that
turned many people off Agfa films, quite unnecessarily.
Another possibility is that up to 1958 films in the US
were rated by the original ASA sytem. This was based on the
Kodak Speed system worked out by Loyd A. Jones of Kodak but
modified. Jones intention was to determine the minimum
exposure needed for good tone rendition, however, when the
system was adoped as a standard a one stop fudge factor was
added so that generally film came out overly dense In fact,
Kodak used to sate in their instructions that film could be
shot with about one stop less exposure if one had good
control of exposure and development.
In 1958 the ASA standard, the forunner of the current
ISO/NIST standard, was changed to a modification of the
second DIN standard. This relied on a fixed minimum density
above gross fog and base density rather than on a minimum
toe gradient as did the Jones method. This made it a lot
easier to measure. The modification was to add a factor of
1.25 to account for the shift in the speed point from the
minimum gradient point. It was found after surveying a great
many films that the difference between the Jones speed and
the DIN speed would be accounted for by this factor. The net
result was that all published film speeds were about doubled.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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--
________________________________________________________________
Bogdan Karasek
Montréal, Québec bogdan(at)bogdanphoto.com
Canada www.bogdanphoto.com
"I bear witness"
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