How was the delta log H of 1.3 decided upon in the ISO "triangle"? I guess I could ask a similar question for the 0.8 delta D. Is there a relationship to the fractional gradient method that led to these numbers? I'm also trying to understand why there was a need to move to this system versus the fractional gradient. Henry (granted, not the best discussion of speed determination methods) writes the fractional gradient method was difficult to use and also that it couldn't gain international acceptance, which led to the ISO method. Obviously this is an oversimplification, but I don't understand why the ISO method is easier to use, other than it having only one gradient. So what? I guess the direction I'm going in here, is in theory I think I prefer a fractional gradient determination. Not sure yet. Is it possible ISO was preferable because it allows films with different toe shapes to have the same speed, which might have satisfied more manufacturers? For me, film speed or an EI is merely a means to ensuring I have good toe contrast. So the thing that is bothering me here is that there are too many ways to satisfy the ISO method. A short and long toe film can both have the same ISO speed.