Even with exposures at 240 seconds, Bond found no increase in contrast with TMY or Delta 100, no increase in contrast beyond experimental error with Tri-X, and less than a one Zone increase in contrast with HP5+ and TMX. This doesn't include the films with least reciprocity failure like Acros. TMY-2 is decent, but isn't particularly outstanding where reciprocity failure characteristics are concerned.
Originally Posted by polyglot
Have you actually tested Fomapan carefully and gotten the N+3 expansion that you describe? If so, I'd love to see the data. Your logic is correct, but that doesn't appear to be the way current films work in practice, based on the data I've seen from Bond and astrophotographers' testing.
As Bond mentions in his article, most manufacturers' data sheets are woefully out of date and inaccurate where reciprocity failure characteristics are concerned.
FWIW, I actually do sometimes use films with greater reciprocity failure to increase exposure times for things like pinhole work.