I once went to the trouble to look up the actual definition of the ASA/ISO value for photographic materials (I think it was in the collection of ANSI documents at Chicago Public Library). Since the true ISO value is a function only of the film, it relates to the "toe" of the exposure curve, since the slope of the curve above that depends on development. Ansel Adams' careful description of the Zone System is consistent with that definition. "EI" generally relates to a midpoint up the curve (where one would normally expect 18% grey), and therefore depends on development. I have no idea how the digital camera manufacturers define the "ISO" value on their cameras, but I suspect it is closer to "EI". If you change the "ISO" setting on a film camera, it only effects the setting on the internal light meter (with or without auto exposure based on that meter).