The last time I shot very expired Tri-X, I tried to use it as if it were new. I primarily shot it at box speed, with just a quick test of overexposure. I had read that expired film should be overexposed, and it makes sense that exposure compensation is needed to account for increasing base fog, but I was curious how the rule-of-thumb applied to Tri-X. From the perspective of someone who knows little about film chemistry, there seem to be at least two dimensions that need to be considered when deciding how to compensate an expired roll of film. First, it seems fair to assume that color film might degrade faster than black and white film due to the additional chemical complexity of those films. Second, it seems reasonable to assume that original box speed will impact how fast a film will degrade. So, I wasn’t sure if the rule of thumb to overexpose one stop for each decade a film is expired applied to Tri-X. After processing the last roll of expired Tri-X, it was clear that 45-plus year old Tri-X needed at least two stops of over exposure.
After shooting this roll of Tri-X, I am convinced that expired high speed black and white film should receive at least one stop of compensation for each decade it is expired. I tested this roll on a low contrast scene at dusk while using filters, but I had another film back with unexpired Tri-X to compare the shots against. And, even at 4⅓ stops of relative overexposure the expired film was still slightly darker than the unexpired film. Perhaps bumping it to 4⅔ would have made them equal. Or, maybe an additional correction factor is needed when using filters such as the red 25(A). A good comparison is hard to make. The expired film has heavy base fog and the total range is compressed. With the equipment I have it is really only possible for me to compare the inverted scans, and that process adjusts image levels.
As I shoot through the the large lot of expired film I have, I will need a formula to help me determine the effective ASA for each roll though. So, based upon my recent experience, I am going to guess at a starting formula and iterate as I learn more. I think the formula should consider the age of the film, the filters being used, and it should skew towards overexposure. Considering those factors, my fist formula is as follows.
exposure compensation = (current year - expiration year) / 10 + filter factor * (1 + (current year - expiration year) / 150)
If I would have followed this formula, then frame 6 above should have been shot at 8⅓ (4.5 + 3.9) stops overexposed rather than 7⅓. Frame 6 turned out the closest to the benchmark, but was still slightly dark, so that seems reasonable. I will try this formula on a few more rolls of black and white, then try some color film. Stay tuned.