Since my preferred B&W film (Fuji Neopan Acros 100) for long exposures is hard to find in 8x10, I decided to try my hand at using Kodak Tri-X 320 for night shots. Because of its steep reciprocity failure curve Tri-X is probably not the ideal film for long exposures, but people have been using it for decades successfully. Surely I could figure it out. I knew that a metered two minute exposure would actually be much longer than 2 minutes. But how much longer? Double it to four minutes? Triple it to six minutes? I started by scouring the interwebs and found this formula attributed to Gadget Gainer:
tr = tc,1*(tm^1.62) + tm
tm = exposure time indicated by the meter tc = amount to be added to indicated time tc,1 = correction to be applied to a 1 second tm tr = exposure time adjusted for reciprocity failure
Wow, I have become adept at working with numbers while mixing different developers and calculating exposures, but that formula is way beyond me. Luckily some smart people on various photography forums had taken the formula and created graphs showing adjusted times for most current films. Of course each person had their own take and experience on the matter and a two minute metered exposure for Kodak Tri-X 320, when you factor in reciprocity failure, was listed anywhere from 6 minutes to 18 minutes.
Honestly, the only way to figure it out is to use these times as starting points and consider the exposure that you are trying to make. You have to experiment and discover the right time for yourself using their experience as a guide. I chose to split it down the middle and use 10 minutes for a metered two minute exposure.
The Brooklyn Bridge and Foggy Lower Manhattan, 8x10 Kodak Tri-X 320 Sheet Film
I shaved about 15 seconds off my normal developing time for Kodak Tri-X in Kodak HC110. Probably could have pulled it out of the tray about 30 seconds earlier. The foreground is a little muddier than I would like, but it's a successful test shot. Next time I will probably use 8 minutes.
For the next shot, I metered it at 8 seconds, which according to the above formula and the experience of others, should be adjusted to anywhere from 16 to 28 seconds. While I was setting up this shot I got a little distracted by some fellow photographers who were interested in the camera and wanted to look though the ground glass. After talking a bit, I said I needed to take the shot. I rechecked the focus, the lens settings, and and exposed it for 8 seconds, forgetting to extend the exposure time to account for reciprocity. However, it came out silky smooth and just slightly underexposed in the shadows.