This started as a MASSIVELY OT comment
about Eirik’s question about a film's speed. As it is a very nerdy question I thought I would do folks the favor and make it a side post, instead of killing this thread!
That way people could ignore it at their leisure (francois, you cant tho!). It has to do with understanding a film’s ‘sensitometric' curve and the relationship between it and the films rated ISO.
Looking at a typical film curve, showing density and Log exposure...
The flat section of the curve is generally regarded as the film latitude, which makes sense as you want the film density behaving in a predictable manner. My understanding is that the rated film speed will for generally in the middle of this flat part of the curve
*.
I found this site pretty informative (hopefully it is also correct):
http://filmshooterscollective.com/analog-film-photography-blog/a-practical-guide-to-using-film-characteristic-curves-12-25 From it gave an equation to convert Log Exposure in lux-seconds into ISO:
LogLuxSeconds = log10 (10/ISO)
If I remember my math correctly, that would mean that
ISO = 10^(1-LogLuxSeconds) (That is intended to read as “Ten to the power of, bracket, one minus logluxseconds, close bracket.”)
The second equation is correct, i believe, if the first equation is correct!
These values correspond for the sample values that the above info gives in a table and the examples given in the above link. They also seem to work when I go look at various curves (as long as the X axis is using lux-seconds).
SO I HAVE 2 BIG QUESTIONS:#1 Is this more or less correct???Am I just fully cooking with horseshit or is this more or less the relationship of ISO to lux-seconds?
My OCD is hitting me bad over this, haha, and I really wanna know if I am viewing these bloody densitometric curves in any way that makes sense!!!
#2 What does it mean for Eirik’s situation?Eirik, I know that you supplied a product data sheet for kodak vision 3 intermediate film, but is this not the film you are wondering about?
http://motion.kodak.com/motion/uploadedFiles/kodakVision52427242.pdfIf that is the case then I would eyeball that that ISO could be calculated at somewhere around the point 1.2 lux-seconds which according to the about formula would would out to be 10^(1-1.2) = 10^-.2 = .6 ISO
That vision three film that eirik was comparing it to looks like it is much more sensitive, however in that it looks like its ISO would be somewhere in the range of 400 ISO: 10^(1- -1.6) = 10^2.6=398 ISO
I also found this site, pretty good:
http://www.olympusmicro.com/primer/photomicrography/filmexposure.html*This is a bit of a gross exaggeration: You can eyeball it or a more specific method from wikipedia follows: Film speed is found from a plot of optical density vs. log of exposure for the film, known as the D–log H curve or Hurter–Driffield curve. There typically are five regions in the curve: the base + fog, the toe, the linear region, the shoulder, and the overexposed region. For black-and-white negative film, the “speed point” m is the point on the curve where density exceeds the base + fog density by 0.1 when the negative is developed so that a point n where the log of exposure is 1.3 units greater than the exposure at point m has a density 0.8 greater than the density at point m. The exposure Hm, in lux-s, is that for point m when the specified contrast condition is satisfied.