Originally Posted by wiedzmin
I'll try to explain this bit. My usual film is 400 TMAX, so I have done lots of tests of that film and the toe of the curve crosses 0.1 density at a fairly consistent place on the x-axis.
So I just assume that is ASA 400.
In the "Hitting ASA Triangle" thread, I found my usual benchmark had 2/3 stop uncertainty. I developed Panatomic-X and compared to the benchmark it indicated greater than 32 ASA speed. Now that is highly unlikely. More likely I never really get ASA 400 from 400 TMAX (Other issues: Neutral Density Filters are non-neutral. Different spectral responses of 400 TMAX and Panatomic-X. Dirty test wedge that I'm not willing to spend $100 to replace right now).
So let's simplify for a moment and ignore the 2/3 stop problem - I can always go back later and fix that.
I have enough 400 TMAX film curves that I can take one that meets the ASA triangle. If I draw a vertical line where that curve hits 0.1 density, all I have to do is declare "That is 400" - everything else compares to that.
Now develop any film and draw its curve. If the toe of the film curve crosses the 0.1 density 0.3 density units to the right, is 1 stop slower than 400.
So you can pick up a brick of unknown Tri-X, develop it "as much as possible" to fit the ASA triangle. Now in relative terms, where the curve meets 0.1 density, you can tell how much slower that film is due to fog or age.
The sensitometer I use has a flash that is 1/100th second. It's like a manual flash, always puts out the same amount of light. So the graphs should compare.