Quote Originally Posted by markbarendt View Post
Like Bill says this is for external meters.

The math is easy, just use the filter factor as the denominator and the film's box speed as the numerator. The result is the ISO number you use to set your meter.

So for Tmax 100; film speed of 100 over a factor of 2 means you set the meter at ISO 50 (100/2=50). For filter factor 5; 100/5=20.

For Tmax 400 and an orange filter; 400/4=100.
I never knew this!!!

No wonder some I've my exposures always seemed off, I always thought filter factors were STOPS...

Kind of stupid, why make you do math instead of just putting the stops on the filter? I'm sure it "all goes back to xxxxxxx reason" it's still annoying and dumb.

Thanks for the info