How do I move from the exposure time from a traditional lightmeter to one that is useful for Pinsta?
Example: Lightmeter: ISO = 100, f = 11, exposure time = 1/2 seconds
What is the equation I need to use to find the exposure time for ISO = 2 and f = 209 ?
To convert f/ numbers divide the Pinhole f/ by the Lightmeter f/, and Square it. (The standard pinhole for a Pinsta 4x5 is f/229
(229/11)^2 = (20.8)^2 = 433 Multiplication Factor f/
To Convert Iso divide the Lightmeter Iso by the Paper
100/2 = 50 Multiplication Factor i
To convert your Lightmeters calculated exposure just multiply those two MF's
For example
0.5 sec x 433 MFf/ x 50 MFi = 10825 sec = 180.5 minutes.
Rounding by a few seconds at this length of expsoure isn't going to make any difference. If you use film, you must take into account Reciprocity Law Failure, this is not required for Direct Positive Paper.
If you want a more detailed explanation, see my profile Instagram https://www.instagram.com/p/CygWQ1SSZVW/