ISO have defined a standard for film speeds. Approximately, it defines the exposure you need to get Zone I at a density of 0.1 above film base plus fog, for a given contrast.(posted 9331 days ago)This is nice. There is an absolute, rock-solid definition of film speed. Anyone who quotes an ISO film speed can be taken to task if they are wrong.
So what do the manufacturers do? They don't tell us the ISO speed of their films. For example, I am not aware that Ilford have published the ISO speed of their new film "Delta 3200 Professional". Because they won't tell us, we have to do our own tests, with our non- standard equipment. For what it's worth, I think it is about 800, the same as "T-Max 3200". Although my equipment and methods are non- standard, they are well within one stop.
It is important to get these results, otherwise we may be fooled by the name into under-exposing by 2 stops.
As far as I am concerned, the ISO definition may be imperfect, but it is the only *real* speed.
One nice aspect of the ISO definition is that although it refers to only one contrast, if we develop for longer (or shorter), we will get greater (or lesser) contrast, but Zone I will stay at roughly 0.1 above fb+f. Hence the expression "expose for the shadows, develop for the highlights".
The other "definition" of film speed is the Exposure Index (EI). There is no standard definition for the EI. Indeed, this is the whole point of it. The EI you use should be whatever you set your meter to, in order to get the results you want (which includes variables like contrast, agitation techniques, thermometer accuracy, water quality, and so on). So if a manufacturer tells me I can use an EI of 3200, or 25000, they mean they can set *their* meters to that value and get the results that *they* want, which may not coincide with what *I* want. To be fair, they usually add disclaimers, like '...if you don't care about the shadows' and 'use these as a basis for your own experiments'.
Sometimes, EI means the exposure that places Zone V at a given density (say, 0.9). At greater contrasts, this gives a higher number than the ISO definition. But it really can mean anything at all. Iflord's definition is 'the speed at which the film can be used to produce excellent images.' Well, thanks for the precision, guys.
So, if someone tells you the "real" speed of a film, ask them if they mean ISO or EI. If they mean ISO, you can use that information directly. If they mean EI, you will only be able to use the information if you happen to use the same metering technique, etc.
Don't get me wrong, the concept of EI is great. When I know my own definition of EI for a film/developer/time/etc, I can set my meter and fire away. But because everyone's definition of EI is diferent, don't expect them to agree.
And this is really the point of experimentation. I agree with Andy and Brian, in as far as the most important thing about film speed is to know what works for you, rather than what the standards say.
Incidentally, my own definition of EI is "the speed that gives me good detail in Zone III, which generally means Zone I is 0.1 above fb+f". This definition works for the way I meter, but would *not* work for high contrast films, where Zone I might be 0.1, but Zone III might be totally blocked out.
As for literature, "round up the usual suspects": A. Adams, "Beyond the Zone System" by Phil Davies, and the Focal Encyclopedia of Photography.