# Exposure Compensation when using filters

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### #1 Hamish Saks

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Posted 29 January 2015 - 07:39 PM

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.

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### #2 Bruce Greene

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Posted 29 January 2015 - 07:54 PM

20
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### #3 Satsuki Murashige

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Posted 29 January 2015 - 08:54 PM

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.
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### #4 Bruce Greene

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Posted 30 January 2015 - 11:46 AM

Oops, i didn't notice that he assumed 2/3stop for the pola instead of 1 2/3..,.
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