Here is a math equation that "fit" the data for this HP5 film given by Ilford:
Corrected exp = ( 0.103333 * X * X ) + ( 2.22143 * X ) - 2.08333 X is the measured exposure in seconds for instance, to find the corrected exposure for 120 seconds (2 minutes): Corrected exposure = ( 0.10333 * 120 * 120 ) + ( 2.22143 * 120 ) - 2.08333 Corrected exposure = ( 1487.9952 ) + ( 266.5716 ) - 2.08333 Corrected exposure = 1752 seconds or roughly 29 minutes I think Benno was saying that Steve Simmons' "Using the View Camera" suggests an exposure of about 28 minutes (give or take 10 secs :-). The equation gives you 29 minutes. One minute exposure when the exposure is that long won't make any difference, be sure of that, besides, with pinhole apertures I rather give a bit more than less, after all I've never had an overexposed pinhole made negative, but I do have many underexposed ones!!. For up to 1/2 minute you have corrected exposures given by Ilford. You could make a table of corrected exposure for up to 5 minutes (this is the max you wanted, I believe) just by plugging different values of "X" or measured exposures in increments of 30 seconds. Hope it helps, Guillermo
