Many people tend to confuse film speed with personal shooting preferences with exposure. The link on page two of this thread is a perfect example of this. Most of the time this is fine since it doesn't really affect the quality of the image. After all, the goal is to produce a usable negative and being pragmatic about exposure is good enough and as far as most people want or need to take their understanding. There are people in today's age of digital photography who have never shot film and have no idea what an f/stop is or how it works, yet they are perfectly happy with the quality of the images they produce. Problems and confusion begin to occur when someone wants to learn more about film speed while remaining in the pragmatic camp.
In truth, film speed changes little with changes in processing. For any given developer, there is in essence only a single film speed for processing between around -1 to +2. This is because film speed is not tied to density. While the ISO standard uses a point of density to determine film speed, the standard doesn't address speed theory. It is only a document of instructions for the determination of film speed. If you want to know the whys, you have to look somewhere else. Hint: The fractional gradient method of speed determination has always been regarded as the most accurate method. The fixed density method of the ISO standard is really just a different way to determine the fractional gradient method for normal using a mathematical conversion known as the Delta-X Criterion. Using a fixed density method for anything other than the defined parameters of the standard will result in skewed results (lower than normal contrast will result in underrated speeds and higher than normal contrast will result in overrated film speeds).
Flare plays in important part in film speed. Without the influence of flare, all films would be one stop slower. And as flare is variable in actual shooting situations, it is impossible to ever precisely place the shadow density. While the ISO testing eliminates flare in the testing procedures, a one stop of flare is factored into the final determination.
The point in which film speed is determined (speed point) is not necessarily the target point in which the shadow exposure is supposed to fall.
The idea of "personal" film speed is more about the defining your personal shooting and metering habits. You can't even say that it is about incorporating your equipment into the equation because there are just too many variables for that to accurately happen. Determining the film's actual film speed requires eliminating all the possible variables that can effect the accuracy of the results. That way you can be certain the results are only about the attributes of the film.
So, what is the true speed of a film? It is the ISO speed rating. How you want to rate the film and expose it is up to you and your personal results and tastes, but don't confuse the two.
If anyone is interesting in knowing what film speed really is, the only two nonscientific authors who understand it are Phil Davis and Roger Hicks. Otherwise, you will need to read the scientific papers from the 1940s, 50s, and 60s, or books such as Photographic Materials and Processes by Strobel and The Manual of Photography by Jacobson.
