It would be nice if there was a formula which could prescribe the proper compensation required for any roll of expired film. I started to naïvely work out a formula, but having only 3 data points at the time the task was futile. So, instead, I decided to compile results based upon a system of rules of thumb. These rules of thumb apply to film with an unknown storage history, and assumes the film was probably not stored properly. The rules of thumb will be updated with the results of each new roll.
I have been using the formula
((YYYY + NN/12)-(ZZZZ + MM/12))/FACTOR and rounding to the nearest ⅓ stop. Where:
YYYYis the current year
NNis the current month
ZZZZis the year the film expired
MMis the month the film expired
FACTORis the number of years needed to drop 1 stop (e.g. 1 stop / 25 years => FACTOR = 25).
Rule of thumb: 1 stop / 25 years
Rule of thumb: 1 stop / 20 years
Rule of thumb: 1 stop / 15 years
Rule of thumb: 1 stop / 14 years
Rule of thumb: 1 stop / 8 years