> > This has probably been discussed recently, but can anyone out there > recommend a good formula/rule of thumb for adjusting exposure for > reciprocity in Ilford films? The technical data on their webpage is merely > a chart, and I am hoping to get more specific information, based on the > experience of the other pinhole photographers on the list.
I found something on the internet about HP5+ and reciprocity failure that had been posted by an Ilford technician. He said that you could figure the adjusted time by taking the metered time and raising it to the 1.48 power! I have been using this formula successfully for about a month. It seems to give good times. I have no idea why this "works" mathematically. I tried to find a similar number for Kodak films, using their chart, to see if there was any sort of consistent logarithmic relationship between the metered times and the adjusted time, but there didn't seem to be. However, I have to shoot HP5+ in my pinhole camera at 3200, for a scene of normal subject brightness range. Also, of course, my f stop is small so I multiply it by a factor of 30 times the f64 metered time. I also found out that there is no need to reduce development time for long exposures with HP5+, even though it's recommended for Kodak films like Tri-X. --shannon
