ASA 400 - 1 stop per 10 years in good storage conditions...(55° basement to fridge) from normal radiation/aging So...
ASA400(1993) - EI200(2003) - EI100(2013)
Now you suspect poor conditions so add another stop and that takes you to EI50
So 100 to 50 is your range, others said this but I thought I would add some info/reasoning to that.
I have some tri-x 400 Aero film (aerial film) and did a speed test with 10 frames as mentioned and actually EI25 seemed best, but if I recall it was from the 70's or 80's
The Noteworthy Ones - Mamiya: 7 II, RZ67 Pro II / Canon: 1V, AE-1 / Kodak: No 1 Pocket Autographic, No 1A Pocket Autographic
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