I have a bunch of different stocks of film that are past dated and that have suffered various degrees of speed loss. Up till now, I have just attempted to re-rate them by taking test shots, and then use them at a reduced ISO based on the tests. But a couple of weeks ago I shot two rolls of Fuji Pro NPS (35mm), not realizing it had lost speed. I developed the first roll and the negatives were a bit thin, like 1 stop under-exposed. Too late to reduce ISO. So for the second roll, I push processed it for a one stop gain. The negatives just came out of the tank; they look normal in density (but a bit grainy under the loup). This got me to thinking. I do not fully understand the chemistry of speed loss through film age. But will it always be best to reduce ISO, or does increased development time also have a role to play? Or a bit of both? My guess is reduce ISO, because push processing is likely to cause an increase in grain and a compromise in dynamic range and maybe some cross-over. But what if the speed loss is in part caused by say emulsion hardening, so it does not accept developer as easily? Anyone have a take on this?