Tollerences are fine, but if all the rating are off by a factor of 2, it's not tollerences that are "at fault".(posted 8165 days ago)It's true enough a film speed is whatever YOU determin it to be for YOUR application, but that doesn't help a lot when your application is for 100 speed film and every 100 speed film (as "marked on the box") actually rates somewhere around 50.
Now if there were a 200 speed B&W film, I'd just go out and buy that, comfortable in the knowledge that it was inflated by a factor of 2 and that I could shoot it at 100 and get what I wanted. However my choices seem to be 100 film shot at 50 or 400 speed film shot at 200!
What's the point of an ISO standard if nobody actually agrees that the numbers it generates are particularly useful? I suppose it's slightly better than film that comes in a box marked "Speed? We don't know, figure it out for yourself", but not by much. Even the suggested developing times given by most manufacturers are off. How many people here shoot at the "official ISO" and develop the "official" time and temperature as stated by the manufacturer?