What Jim and Noseoil are really talking about is shadow placement and not film speed. If you're interested in film speed, I happen to have an article in the current issue of PHOTO Techniques that deals with it. It's titled Flare and Accurate Film Speeds. My premise is the discontinuity between the ISO speeds and speeds from Zone System testing originate from a fundamental misconception within Zone System methodology. If you follow the traditional ZS method of film speed testing, you will produce speeds more in line with the pre 1960 film speed standards. One of the basic premises of the article is that the film speed point isn't necessarily the point of desired exposure, but only a point to define the film speed (explained in more detail in the article). Consider, Zone I is four stops under the meter reading or meter calibration point, while the ISO speed is determined at 3 1/3 stops under and also assumes a certain amount of flare. That's a 2/3 stop difference between the two methods.