----- 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 netresult was that all published film speeds were about doubled.
--- Richard Knoppow Los Angeles, CA, USA dickburk@xxxxxxxxxxxxx ============================================================================================================= To unsubscribe from this list, go to www.freelists.org and logon to your account (the same e-mail address and password you set-up when you subscribed,) and unsubscribe from there.