Response to More on film speeds

Thanks Conrad. It seems that everything was on that page, except what I wanted to know!
I did a more accurate analysis of manufacturer's sensitometry curves, last night, and it seems my guesstimate of K=1.5 might be pretty close.

I converted the Log H value of exposure which gave 0.1D +B+F on the makers curves to linear lux-seconds. Then I applied the formula 1/(lux-seconds*1.5), and this gave me realistic EI figures for most of the films I looked at.
For example: T-max100 in D-76 (normal time) was giving an EI of 80; Fuji neopan 400 in D-76 was reading EI 325; and TMY400 in D-76 was giving an EI of 300.
All these figures are very believable, and tally pretty well with my own speed ratings, and those that others have posted on this forum.

It's my belief that by using the maker's published curves, it's possible to get more realistic speed ratings than the ISO figure. Ignore anything it says on the box, and just take the exposure required to get 0.1D above base and fog from the relevant published film/developer curve.
This doesn't work with Ilford films though, unfortunately, since they still label the exposure axis in the antiquated Relative-log form, and not in absolute Log Lux-seconds.
If David Carper is reading this, Ilford please note!!

[ Previous | Next ]