When using my Lee Big Stopper 10 stop neutral density filter I noticed several times that the calculated theoretical exposure time always resulted in slightly underexposed shots. Lee Filters themselves mentioned that the filters are within a certain tolerance and do not darken to exactly 10 stops.
I have calibrated my Big Stopper and came to the conclusion that my filter darkens exactly 10.5 stops. This seemingly small variance has a significant impact in practice. I will give you a short example. In a low light evening shot where the right exposure time is 1/8th of a second, the calculated prolonged exposure time with a 10 stop filter is 02:08 minutes. With my filter darkening 10.5 stops i have to expose for 03:01 minutes to get to the same histogram. The effect is even greater (difference in seconds) at longer exposure time without filter.
I didn’t want to find the correct exposure times by incremental shots since it can take some time and i might miss the best light of the day. So i decided to write a calculation table where you can set the real f-stop value of your filter and adjusted exposure times will be given automatically.
Of course the table can also be used “backwards” for calibration of the filter.
Here is the procedure for your own filter calibration:
You can then print the table, stick it to your filter box where it will be always available on location. The extent of exposure is very broad and in practice one will use only the central area. By deleting individual rows the table can be customized. Of course this table is not just for the Lee Big Stopper but also for any other ND filter like Formatt Hitech, Haida or B+W . The exposure table is created in Excel and can be downloaded here: DOWNLOAD
If you prefer using an app for calculating the exposure time i would recommend Exposure Guide. More information about this app can be found here.