----- Original Message ----- From: "Rune Tallaksen" <tall...@alfanett.no>
> Can anybody help me with how to determine exposure when correcting for > resiprosity failure when the measured esposuretime exceeds that of the table > from the filmvendor? > > For instance when using Ilford HP5 Plus, the film data sheet gives the added > time for resiprosity failure until initial measured 35 secs(corrected for > pinhole f stop). How can I calculate the "correct" exposure time beyond this > point? Tiral and error until now has not been satisfactory for me. One way to do it is by extrapolating the curve Ilford is giving you. You can do that mathematically or graphically. I usually do it graphically. Take the Ilford curve and reproduce it on a piece of paper, extend the "X" or horizontal axis to read the maximum "uncorrected" time you want and then extend the curve with pencil trying to follow the same tendency, then is just a matter of extending the "Y" axis and graduating both axis. I just did that very quickly and the resulting extended curve gives me the following values, for 60, 90 and 120 secs of uncorrected exposure: 60 secs uncorrected = 475 secs corrected 90 secs = 925 secs 120 secs = 1900 secs Once you have the extrapolated curve, use it and if you get consistently somewhat overexposed film, that means the curve was done a bit to steep. If film is underexposed consistently then the curve needs to be re-done a little more steeper. One thing for sure, extrapolating the curve would give you better reciprocity corrections than "trial and error". Hope it helps. Guillermo
