It is a means to an end, but that doesn't mean it's arbitrary. If done according to the ISO Standard, the speed point can yield useful information. It defines the relationship between the meter exposure and the shadow exposure, it defines the local contrast as well as determining the point of useful minimum exposure (0.29 log-H units to the left where the gradient is 0.3x the average gradient).
Originally Posted by Michael R 1974
This is what I mean about questioning the steps in any methodology. Most methods are just variations of the basic Zone System approach where the speed point is unconnected with the three elements list above. While the 0.10 density may have come from the ISO speed standard (the ZS use of 0.10 probably was derived from DIN or the early British standard), it has been misinterpreted or simply impoperly applied.
Using the ISO speed standard we know the speed point is 1.0 log-H units to the left of the metered exposure point. According to many popular methods, the speed point is four stops down or 1.20 log-H units below the metered exposure point. Obviously the two methods will produce film speeds typically differing by two-thirds of a stop. This is why Zone System testing consistently produces EIs 1/2 to 1 stop slower than the ISO speed. While the 1.20 log-H method will produce quality negatives and will create perfectly acceptable EIs, it shouldn't be regarded as a reliable way to determine film speed. Of course, if the additional exposure from the lower EI matches your style, why not save the time and just use the ISO speed and make the 2/3 stop adjustment. Afterall, what's the point of doing the test if it's not going to produce a reliable number.
Most of the authors won't explain this difference (mostly because they aren't aware of it), so when shopping around for a non-ISO testing method, understanding the theory is important. Just because someone wrote it down doesn't mean it's true. This doesn't mean you shouldn't use a non-ISO method. I'm just suggesting to use it knowingly.