Originally Posted by

**Rafal Lukawiecki**
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 = log_{2} (exp/2.5)

so, log_{2} 1.13/2.5 = -1.15