When the HECK did ASA become ISO??

[JW Dewdney]

Okay- so we used to have this system. ASA film numbers. Typically, 100 (normal speed), 400, etc. etc... and we had a european system called ISO - where 100 ASA translated to 21 degrees ISO. Every doubling or halving of ASA speed would result in adding 3 or subtracting 3 to the ISO scale. Logarithmic scale. Simple enough. But in recent years, everyone's referring to what was PREVIOUSLY ASA as ISO. What gives? Sorry if I'm not on the bandwagon - guess I stopped paying attention years ago.

Can someone also give me the lowdown on granularity indices? According to what I've read/heard, Fuji NPS should be a much finer, more detailed film than Velvia 50 - which makes no sense to me.

Thanks so much, if anyone has any feedback opinions on these issues...!

[Oren Grad]

That's not quite right. The linear-scale American standard was ASA, while the logarithmic-scale European standard was DIN (Deutsche Industrie Norm). The new international ISO standard formally combines them in this sort of notation: ISO 100/21°. However, it's common practice to omit the logarithmic element, making it look as though the ISO system is just the same as the old ASA system.

[Ted Harris]

More arcane detail to follow on behind Oren. ASA stands for the American Standards Association (now American National Standards Institute) which, along with the National Institute of Standards and Technology (NIST .. former National Bureau of Standards), feed into the International Standards Organization ((SO). ANSI is industry based and private and sets voluntary concensus standards, NIST performs similar functions on an official basis for the USG; ISO sets international standards. It is not just film speed but everything from baseline measures of weight and length, etc. to composition of materials and accounting standards.

[Oren Grad]

The current version of the applicable ISO standard is:

ISO 6:1993 Photography -- Black-and-white pictorial still camera negative film/process systems -- Determination of ISO speed

[Oren Grad]

And on the color films, not only do measured granularities correlate only approximately with subjectively perceived graininess, my recollection is that there are technical reasons why measured granularity indices of negative and slide films cannot be directly compared. I'd have to do some digging to locate a specific reference on that for you - perhaps someone else here is more on top of the details.

[Helen Bach]

ANSI is the organiser of the ISO technical committee (TC 42) that prepares photographic standards, so ANSI is still heavily involved.

Reversal colour film speed: ISO 2240:2003
Negative colour film speed: ISO 5800:1987
Digital cameras: ISO 12232:2006

Best,
Helen

[Helen Bach]

A single number for RMS granularity doesn't tell you much. It would be better to show R, G and B granularities over the density range of the film. That would tell you a lot more than the single number, because granularity varies with density and layer. Fuji give the RMS granularity for a density of 1.0 above DMin for both reversal and negative colour films.

Contrast affects the appearance of graininess - so the granular appearance of negative film will be increased by most printing methods and decreased when printing reversal film. Assuming a typical print density range, the density range of negative film is magnified by printing (say 1.2 to 2.0), while it is decreased when printing reversal film (say 3.0 to 2.0).

Does that help?

Best,
Helen

[Ted Harris]

Laughing ..... JW, I hope that Oren's, Helen's and my posts have not drivenyou over the cliff .... although all are totally correct.

One more point on RMS granularity. The single number the manufacturers give you and Fuji makes so much out of in its marketing hype is still useful in comparing one film to another. OTOH today's films are all sooooo good in this respect.

[Kevin Crisp]

JW: This happend a long time ago, I'd guess as far back as 1980.

[Helen Bach]

The official birth of ISO film speeds could have been even earlier. TC 42 was in existence in 1972, and ISO arithmetic/logarithmic speeds were mentioned in the 1974 ISO standard for light meters that replaced the 1971 ANSI standard.

Best,
Helen