Quote Originally Posted by albada View Post
You'll laugh at this: I guess based on where various films hit .1 over B+F!
That's their ISO speed-point, and I know where that should be on the log E axis. Of course, developers shift that point, and there's no guarantee that the films themselves hit it exactly. But this gets me close. All my work has been comparative, usually comparing with XTOL's curves, so I haven't needed accurate log E numbers.
Mark, there is a way to quantify those results. It's not perfect as you can't be certain of the metered exposure, so any variance of the film speed can't be attributed with certainty to any one factor. The basic idea is to know the value of the metered exposure. Once you know this, you can determine the exposure value at what ever additional stops is given for the test. Next, know the target exposure at the speed point for the the speed of film being tested. You can then calculate what step tablet density that should fall at for a given speed with your chosen exposure value. The attachment breaks this technique down. It's from an analysis of a method proposed by Schaefer, but it should be applicable to your situation.

BTW, if your interested in learning more about the ISO speed point and what it means, check out the thread http://www.apug.org/forums/forum48/1...-bill-etc.html ,ISO speed determination constants - question for Stephen, Bill etc.

Schaefers exposure.jpg