For the first time at the weekend I stacked a 10 stop and a 6 stop filter. Alan R was using his 16 stop Hitech. My Lee filter combination required a much longer exposure than Alan’s. In fact I had to double the ISO to bring the times down to the expected 300 seconds. I contact Lee and they said
"My guess is that it's possible that both of your filters are slightly darker than the stated figure - we have a tolerance of +/- 0.5 stop on our stoppers, so the 10 will be between 9.5 and 10.5 stops - the same with the 6 stop. If both filters were even 1/3 stop dark, adding them together could add 2/3 stop and + 1 stop means double the exposure time or double the ISO. It’s the sort of thing you don't notice on one filter but compounding the two for long exposures kind of emphasizes the situation.”
So that explains that then........possibly.
"My guess is that it's possible that both of your filters are slightly darker than the stated figure - we have a tolerance of +/- 0.5 stop on our stoppers, so the 10 will be between 9.5 and 10.5 stops - the same with the 6 stop. If both filters were even 1/3 stop dark, adding them together could add 2/3 stop and + 1 stop means double the exposure time or double the ISO. It’s the sort of thing you don't notice on one filter but compounding the two for long exposures kind of emphasizes the situation.”
So that explains that then........possibly.
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