Hi Pentaxuser. My method is entirely logistical or practical. I regard any realistic film speed in relation to what is necessary to begin resolving steps of shadow value, or lifting them up off the toe threshold enough, to equate to reasonably discernable gradation in the print itself, or something equivalent in a densitometer reading. Some people call that personal ASA, some talk Zone System jargon, whatever. But it has to work. I encounter a lot of high contrast situations. I need to know at exactly what point the film reasonably launches off the toe, and at what point it begins shouldering of at the top. This is all specific developer dependent, of course.
This past week I've been doing my first shots with Acros II 120 film. I already knew it had a slightly steeper toe than the original version, which I always shot at 50. Today the contrast was generally moderate due to an amount of coastal fog, so I'm confident I bagged full tonality even shooting it at box speed 100. But on that same roll I have a couple shots made in relatively high contrast settings, which I also shot at 100 just to see how they will turn out. When I get up to high altitude usage, then it will be critical to know the real-world parameters. I've done lots and lots of Acros 4X5 shooting at high altitude, but it's always been at 50. The ONLY med speed film I trust at box speed under those conditions is TMX100. So this is an appropriate time to fiddle around with the new Acros II and see just how much it differs from original Acros, and if it comes closer to the long scale of TMax than its previous version. It is also slightly less blue sensitive, another improvement in landscape applications.