I'm looking at the Fomaspeed VARIANT paper's technical sheet. I want to learn what is the ISO speed of this film (I need this to evaluate the exposure time in a pinhole camera) - and the spec says (see second page):
Filter | Contrast grade | ISO R speed | ISO P range | Lengthening factor ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 00 | special soft | 160 | 200 | 2.4 0 | extra soft | 130 | 200 | 2.4 1 | soft | 110 | 200 | 2.4 - | special | 100 | 500 | - 2 | special | 90 | 200 | 2.4 3 | normal | 70 | 200 | 2.4 4 | hard | 60 | 100 | 2.4 5 | ultra hard | 50 | 100 | 2.4
(table drawn manually, sorry if it's messed up on the mobile device).
Question: what's the ISO rating for this film when used in a pinhole camera? What's the difference between ISO R speed and ISO P range?
Side question: what's the lengthening factor?
Answer
ISO (R) is not a speed number, it is log exposure range required to give the full tone (that is, full density range). The higher is the ISO R number, the softer is the paper. ISO R = 160 means density range = 1.6, or log2(10^1.6) = 5.3 stops. You may see it as "paper dynamic range under standard development". ISO R is used because contrast grade is in fact different between different manufacturers. One manufacturer's 3 is the other manufacturer's 2. ISO (P) is paper speed, and it is loosely related to film speed as 1:80, that is ISO P = 160 is about the same as film speed of ISO 2..3. For multigrade paper the complication is that it changes both R and P numbers depending on the light spectrum. That is why for shooting, graded paper is preferred over the multigrade. Lengthening factor is the factor for the exposure time for the paper to reach the ISO numbers.
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