ND filters

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dunerider

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Can someone explain how ND numbers relate to f stop numbers when buying filters?

I bought a cheap set of filters recently where the ND2 was 1 stop and the ND4 is 2 stops. However i am now looking at a ND3/10 stop filter.

I had assumed the ND meant how dark the filter was and thus the number of f stops, but this doesnt seem to follow.
 
The "ND" part simply means "Neutral Density". How dark they are is given by the rest of the name. There are two ways of expressing this: in terms of stops or in terms of density. As you've seen, 2 means 1/2 of the light gets through, so one stop; "4" means 1/4 or a quarter, so 2 stops increase are needed. Density follows a logarithmic scale, and every increase of 0.3 doubles the density. So ND2 is equivalent to ND 0.3, ND4 to 0.6 and so on.

10 stops is given by 2 to the power 10, which comes out at 1024; in density terms this is 0.3 times 10, which is 3.

So specifying ND3/10 is simply using both systems in one breath as it were - density 3, stops 10. In the same way, ND2 would be ND0.3/2 (one stop increase).
 
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I would suggest you check out the following site for an explanation of how filters work, their variances, how and when to use them.

It is of course fully Lee Filters focused but much advice is relevant to any filter brand:-

www.leefilters.com
 
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