Quote Originally Posted by Chan Tran View Post
Make a reading at your box with the flat diffuser. Count the number of stop from f/1.0 for example if it displays 5.6 3 then 5.6 is 5 stops from f/1.0 plus .3 then it's 5.3
The Lux.second is (2^5.3)*2.5=98 lux.sec
Thank you! I was looking for the formula, I'm glad it is so simple.

On another note, I have been testing the repeatability of my flash with a meter, which shows down to 0.1 of an f/stop. It seems to depend on the selected power output. At lower outputs, it is less precise, hitting up to +/- 0.2 f/stop third of the time. At a higher output it is much more accurate, hitting the same level 5 out of 6 times, and deviating -0.1 once in six.

I wonder if a tungsten bulb controlled by a spare enlarger timer would be still a better choice in the long term, but perhaps I have what I need to complete the re-test now.