Originally Posted by
David A. Goldfarb
Did you shoot PL100 from the same source before and test it at EI 100? If not, it may just be that PL100 is EI 50 with whatever developer/time/temperature/agitation you are using.
I've found that Fortepan 400 can drift as much as a half stop between batches, and John from J&C thought it was likely a storage issue, since I had obtained the different batches from different suppliers, and that is a film known to be sensitive to storage conditions.
I would think that an ISO 100 film would be less sensitive to storage problems than an ISO 400 film, but if the film is maybe EI 80 in a certain developer and then has lost a little speed due to storage, and maybe is off a bit to begin with due to quality control, it wouldn't be too far to get to EI 50.
After noting the speed drift with Fortepan 400, whenever I buy a new batch of one of these films, I usually run a simple test against the previous batch--one scene, one film on one side of a holder, the other batch on the other side, expose in sequence, and develop together, and if I notice a speed difference, I write the new speed on the box, possibly with further testing.
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