----- Original Message ----- From: "Marc James Small" <marcsmall@xxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx> Sent: Tuesday, September 25, 2007 9:23 AMSubject: [rollei_list] Re: OT Infared film and shooting with it.
At 11:35 AM 9/25/2007, Eric Goldstein wrote:>The manufacturer's rated speed of the film is its ISO... there is no
>more ASA... > Har. Har. Har. "ISO" is just a combination of DIN and ASA. That is, film was once listed as either 21 DIN, for example, or as 100 ASA. Today, it is listed as "100/21 ISO", but even the Nikon users on the List can decipher this as "21 DIN" or "100 ASA". It is NOT rocket science, so to speak. So, yes, it remains proper to speak of ASA speeds, as these remain part of the ISO system. Marc msmall@xxxxxxxxxxxx Cha robh bàs fir gun ghràs fir!I don't want to reiterate my rather long post about this. ASA is the American Standards Association. This was renamed the American National Standards Institute, probably because people can't stand simple names. The ISO is an international association of national standards groups and attempts to co-ordinate standards. An ISO standard is also an ANSI standard. The DIN, which is the German association for establishing industrial standards, is also a member of ISO. The original film speed standard adopted by the ASA in the United States, in 1943, was a modified version of a method which had been developed at Kodak and used internally there for several years. The method was devised by Loyd A. Jones after many years of research on determining the _practical_ exposure of film for pictorial use. Jones's method required measuring intercepts of a double slope and evidently was not very simple in practice. Jones idea was that the minimum exposure point should be at a high enough contrast (or gamma) point on the toe of the film curve to provide for some shadow detail. His research found that tone rendition of prints improved until the negative exposure reached this point but that there was little detectable change for greater exposure. He chose to determine minimum exposure because film tends to become more grainy with density and also to loose some sharpness. Unfortunately, the original ASA version of this included a two times safety factor so that negatives were inevitably too dense thus destroying the purpose of the standard. In 1958 the ASA reviewed the method for determining speed. It examined the method used by the DIN at that time. This was _not_ the original pre-WW-2 DIN method but one which measured a speed point determined by a minimum density above gross fog and support density. The ASA conducted tests of a great many emulsions and found that a fixed multiplier of this minimum density point would result in practical aggreement with the speeds determined by the Jones Mimimum Gradient method but was much easier to measure reliably. The ASA adopted this system with the fixed factor (about 1.25) and eliminated the two times safty factor previously applied to the Jones speeds. The practical result was the doubling of all published film speeds. Film speed is still given in both ISO and DIN even through the two are identical methods because ISO speeds are stated in an arithmetical series and DIN speeds in a logrithmetic method. This is to match existing light meter calculators. The ISO/DIN method specifies the exposure range and density range to result from it, thus it specifies the contrast or gamma of the negatives. The last change in the standard eliminated the specification of a standard developer, however the developer must be specified along with the resulting speed. If a film is developed using a different developer than that used for the measurement, or is exposed and developed to a different contrast index (or gamma) the ISO/DIN speed no longer applies. However, the ISO/DIN speeds still give at least a rough estimate of the differences among films.
--- Richard Knoppow Los Angeles, CA, USAdickburk@xxxxxxxxxxxxx
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