What ISO should I use for Fomaspeed Variant paper in a pinhole camera?
Asked 5/2/2015
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I’m trying to use Fomaspeed Variant variable-contrast paper in a pinhole camera and want to estimate exposure. The datasheet lists contrast grades with ISO R speed, ISO P range, and a lengthening factor, including an unfiltered row. Which value is relevant for camera exposure, especially with no filter? Also, what is the difference between ISO R and ISO P, and what does the lengthening factor mean?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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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.
Originally by user28381. Source · Licensed CC BY-SA 4.0
user28381
11y ago
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Generated from our catalog & community — verify before relying on it.
For camera exposure, the relevant value is ISO P, not ISO R. ISO R describes the paper’s exposure range/contrast behavior under standard development; it is not a camera speed rating. Higher ISO R corresponds to a softer paper grade.
ISO P is the paper speed. From the answers provided, paper ISO P values are much slower in practical camera use than film ISO numbers; for example, ISO P 160 is roughly comparable to about ISO 2–3 film speed. So for pinhole work, expect this paper to behave as a very slow material.
With variable-contrast paper, both ISO R and ISO P can change with the spectrum of the exposing light or use of contrast filters. That’s why the datasheet gives different values by filter, and why graded paper is often preferred for camera use.
The lengthening factor is the exposure increase needed when using those filters versus unfiltered exposure. A factor of 2.4 means multiply the exposure time by 2.4 when that filter is used.
For unfiltered use, use the unfiltered ISO P entry from the datasheet as your starting point, then test because paper in a pinhole camera often needs practical exposure adjustment.
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