I've read about the process in Kodak's publication 'Basic Photographic Sensitometry Workbook'.
My understanding of the process for black and white film, using their example, is that you:
- Find the Dmin value & add 0.1 - call this A
- Find the antilog of A
- Then film speed = 800(mililux seconds) / antilog of A.
Dmin = 0.8
A = Dmin + 0.10 = 0.9
Antilog of A = 8
Film speed = 800 lux seconds / 8 = 100
So an ISO rating of 100.
However, colour negatives have three layers of emulsion that are sensitive to different colours of light. Looking at sensitometric curves for a particular film, tends to reveal that each colour layer has a different Dmin value.
So how is the ISO calculated in this case?
The reason I ask, is I have a 100' roll of Kodak Vision 3 5254 digital intermediate film (I know not meant for shooting on), but of like to figure out the approximate iso value & bulk load it into 35mm cassettes.