Originally Posted by Grainy
I have one question, but it doesn't need a new thread. I metered f/22 4 sec. iso 5. After a lot of testing of different exposure times I ended up using 8 minutes exposure time when metering f/22 4 sec @iso5 to get a decent exposure. Is it correct that I need to calculate my exposre using about f/2800 iso5?

I read somewhere that the iso for paper is about 5, but it doesn't make sense when I needed 8 minutes exposuretime.
Not sure I am reading the question correctly -- was this with a pinhole camera? Pinhole cameras typically have an aperture up around f/200 or smaller.

The typical way to deal with pinhole exposures is to make a chart using the shutter speed from a reading for f/22, then multiply it by a factor that relates the pinhole f-stop to f/22. For an actual pinhole aperture of f/220, the exposure relative to the exposure at f/22 would be the ratio of 220/22 squared (I made it easy for myself ) = 100. As such, a reading of 4 seconds would be multiplied by 100, 400 seconds, is 6 minutes, 40 seconds which is in the general order of magnitude of 8 minutes.

With film, these long exposures are affected by reciprocity failure, often quite drastically, as in requiring 3 or 4 times the metered length. For that one can make charts to include the reciprocity compensation. There is a Windows program called Pinhole Designer that can do some of these calculations and generate a chart for a number of films.

A little more detail about what you were actually doing might help.

DaveT