To stop the smart asses among us I know the film speed is printed on the box.

Can someone please explain how to find the actual film speed. I thought I had a handle on it but as I read more I am definately not getting it. How can a developer increase a film's exposure speed? How can BPF 200 go from Ei200 to EI320(as I read in a post on photo.n#$)in rollo pyro? As I am finding out quickly with an increase in format the increase in cost is pretty much proportional.

This is what I understand. Posted film speed is a starting point and, like exposure, dictates the amount of lite needed to record the average light intensity. slower speed equals denser neg, higher speed equals thinner neg.

Anyone shoot BPF 200, and what speed do you use?