I learned the hard way that Efke 100, as well as other old-fashion films, have severe reciprocity problems. Surprising to me is that Efke 25 doesn't, and may even be a good choice in low-light situations. For example, Efke 100 at 100 seconds requires an exposure of 20 minutes ; whereas Efke 25 only needs a 2/3 f-stop increase. Not sure about the math or science; but it does suggest that some so-called slow films may be more responsive to low light levels.