This is a question we all get at one point or another: how to calculate a film's speed when the only information available is its characteristic curve?
Well, this had me dig through some old books for the answer. It came from Hollis N. Todd and Richard D Zakia's classic volume: Photographic Sensitometry - The study of tone reproduction. A book that is great for both the amount of information and its capacity to generate splitting headaches!
While the information in it is quite something, I can't vouch for its clarity so I decided to take the better part of the day to try and clarify things up and do a step by step description of the process used.
Now, over the years, the method used for calculating sensitivity has changed considerably. The original ASA rating method began being used in 1943 and was subsequently changed in 1960 in order to better match the European methods of measurement. Even though we now use an ISO rating, the measuring method is quite similar to that of the modern ASA rating.
Here are some slides describing the process. The curve used is the one from Kodak Vision Color Intermediate film (5242, 7242, 2242, 3242). For the calculation you will need a full page enlargement of the film's characteristic curve, colored pens and a ruler. You can also use a drawing program like I did. It will simplify the addition of fairly precise scale marks.
Now in the technique presented, it is asked that we calculate the density range, something that once the steps completed I discovered is not really necessary.
Edit:
I've now produced a simplified technique that is based on Kodak documentation and appears to be much more simple, precise and useful.
I've now eliminated the part where the density variation is calculated to verify is the development is adequate since we can assume that the companies producing such data take every precaution in making sure that their documentation is correct.
Thank you.
