The "factor" would be the exponent in the equation, not a straight multiplication factor, hence the curve.
Ilford pretty much uses one graph for all their films so I wouldn't rely too much on them. Kodak's publications are similarly murky and/or incorrect.
Some of the more useful data I have seen comes from tests performed by Howard Bond on several films (unfortunately FP4+, one of my favourite films was not one of them):
The chart is at the end if you don't want to read through the entire thing, but it's worth the read.
Based on my experiences with T-Max 100 and Delta 100, Bond's results work quite well. I did my own tests and came up with similar (although not identical) results.
Last edited by Michael R 1974; 01-11-2012 at 04:01 PM. Click to view previous post history.