I'm trying to calculate the needed exposure to place step 28 of my 31-step Souffer, 2.6D, just above fb+f. Could someone confirm if I got my numbers right? I gather that:
Opacity of a density of 2.6 is 10-2.6 = 0.002511
Exposure necessary at film plane is 0.8/ISO so 0.8/320 = 0.0025 lux s
Exposure metered above just above the Stouffer needs to be 0.0025/0.002511 = 1.13 lux s
Looking at the earlier given formula to convert a light meter f/stop reading to lux s:
Number-of-stops-to-right-of-f/1.0 = log2 (exp/2.5)
so, log2 1.13/2.5 = -1.15
which would suggest my meter ought to indicate an f/stop of 0.7, a surprisingly low amount of flash exposure, which it may not be able to read, I would think. I suppose I could just keep reducing down to f/1.0 and then fiddle with a filter of some sort to reduce the flash output even further and perform a test with film.
I plan to have a go at it today, so please shout if you feel I got it wrong. Thank you.