Hamish Saks Posted January 30, 2015 Share Posted January 30, 2015 I was wondering how to calculate the ASA/ISO/EI on a light meter when using certain filters. For example my stock is 250 ASA, and I'm using an ND9 + a polarizer, this means that I lose 3 stops + 2/3 of a stop. What would my ASA then be set to? I'm not sure how to work this out. I understand that I can just take it away from the F stop reading, however, I'd like to be able to just set the ASA on the light meter and then not have to worry about that maths, and just adjust whenever I swap out any filters, rather than doing the maths every time I take a reading. Link to comment Share on other sites More sharing options...
Bruce Greene Posted January 30, 2015 Share Posted January 30, 2015 20 Link to comment Share on other sites More sharing options...
Premium Member Satsuki Murashige Posted January 30, 2015 Premium Member Share Posted January 30, 2015 Minus one stop is 1/2 the amount of light, so it is 1/2 the ASA number. For example, with 250 ASA and an ND 0.3, your new ASA rating would be 125 ASA. There are also 1/3 stops in the ASA scale, so between 125-250 ASA would be 125/160/200/250. There are some rounded off numbers as you get close to the single digits but you get the idea. Actually, light loss from a normal Circular or Linear Polarizer is 1 1/2 to 1 2/3 stop, unless it is a One-Stop Polarizer. So your total stop loss would be -4 2/3 stop. If you swapped the ND 0.9 for 0.6, then you would be back to - 3 2/3 stop. 250 ASA - 3 stops = 125/64/32 32 ASA - 1 2/3 stop = 16/10 So you would be down to 10 ASA with N0.9 and Pola. Link to comment Share on other sites More sharing options...
Bruce Greene Posted January 30, 2015 Share Posted January 30, 2015 Oops, i didn't notice that he assumed 2/3stop for the pola instead of 1 2/3..,. Link to comment Share on other sites More sharing options...
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