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:
YYYY
is the current yearNN
is the current monthZZZZ
is the year the film expiredMM
is the month the film expiredFACTOR
is 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 / 25 years
Rule of thumb: 1 stop / 25 years
Rule of thumb: 1 stop / 20 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