Originally Posted by Joe VanCleave
I've no link to a chart, but I'll instead describe the method I use to determine exposure with pinhole. I use a handheld light meter that goes up to F128. I set the meter to the exposure index that I rate the film or paper to, then meter the scene, then reference the recommended exposure time opposite F128. I then use this formula:

Tc = Tm * ( Fc / 128 ) ^2

Where:
Tc = corrected exposure time
Tm = metered exposure time at F128
Fc = focal ratio of pinhole camera

If you're shooting with the same camera all the time, the values for "( Fc / 128 ) ^2" become a constant that you can write down and carry with you, and then just multiply this constant by the metered time at F128.

If you don't have a light meter that goes all the way to F128, you can meter a scene for F16 and just plug "16" in place of "128" in the formula.

NOTE: This formula does not take into account reciprocity failure, which is common with most B/W films, but less so with Fuji Acros, and essentially nil with paper negatives. There are other formulae for compensating for reciprocity failure, which would do you well to research. But in general, exposures from 1-10 seconds I double the calculated exposure time.

~Joe
Is "focal ratio" the same thing as focal length? The rest makes sense to me. Thanks for the formula, I'll definitely try that out next time. I'm using paper negatives so I don't think I have to factor in reciprocity failure.