From my limited experience shooting slide film, I can see how it could make a difference in exposure. But what about b&w negative film? I’ve thought about this before, rating my FP4 at 100 instead of 125 just because it makes it easier to translate the numbers from my light meter to my camera. And, I got perfectly workable negatives. I started thinking about this again reading people say they’re exposing Fomapan 100 at 80. Does it really make that much of a difference? I was planning on doing a film speed test on this particular film and was wondering if it would even be worth it to use this speed, or just shoot at 100 and 50.