Film speeds- arbitrary numbers?

[joshverd]

Hello,

I'm interested to know if there is any mathematical relationship between film rating and f/stop / shutter speed combinations. Why is 200 speed film called 200?

We all know how to expose them, we just punch numbers into our light meters. But, are these numbers arbitrary or do they give insight into film sensitivity in some way other than our accepted conventions? I imagine there is some historical reason.

Also, how does one find the "actual" speed of the film? Is it just by visual assesment, or is there some way to do it with densitometry, etc...

 

[jstraw]

http://en.wikipedia.org/wiki/Film_speed#ISO_film_speed_scales

 

[jim appleyard]

See Roger Hicks first post, it's the best I've read and sums it up nicely:

(there was a url link here which no longer exists)