After various field tests I've always had to downrate the ISO rate. However this varies with film type and developer combination. I've found the T-grain films to be fairly close to their published ratings. I now only use Delta 100 (both 4x5 and 120) which I rate at 80 (T-Max 100 was the same) and dev. in either Pyro or DiXactol (single bath).(posted 8714 days ago)