I like Bergger Pancro 400 in 4x5 and became curious about how it behaved with long exposures. Now that it is available in 135, I am able to expeditiously and affordably experiment.

Knowing that most films follow an exponential curve, I set up a target in dim light and shot a few rolls of 135 at various exposure times to determine the "right" exposure. I tested this method on FP4+ (for which Ilford has published power law reciprocity characteristics, thanks Ilford!) beforehand.

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I controlled exposure of the target using a tungsten fresnel lamp and CTB+ND gels. The "meter" I used for this was a Canon 5d Mark III (which is my typical long exposure meter). I first determined that the digital camera exhibits "correct" reciprocity across shutter/aperture/ISO settings, and would meter by taking a test shot of the target at high ISO, wide open, followed by a series of exposures on Pancro 400 135 using a EOS 1N with exposure times bracketing Bergger's guidance (e.g. +1.3 stops for 10-60 second exposures). "Correct" exposure was based on negative density compared to a short-exposure (1/4 sec) sample. As expected, the film's reciprocity characteristics follow a power law.

The result was a correction factor of 1.36 (i.e. Corrected Exposure Time = Metered Exposure Time ^ 1.36), determined using a least-squares linear regression of log exposure time.

Pancro 400 appears to have at least a stop of latitude in both directions, which makes Bergger's guidance totally decent for exposures of metered time less than c. two minutes.