Info on B+W ND filters

[unsharp]

I'm about to order B+W 39mm ND filters from B&H via a NY visiting friend.

To not make a blunder, could someone just confirm that filter factor 2 and 4 reduces light by 2 steps and 4 steps respectivly? By looking to the product info Im not totally convinced it's this easy.

Anders

 

[richard_l]

Not quite that easy. A filter factor (FF) of 2 reduces light by 1 stop. FF 4 by 2 stops. FF 8 by 3 stops. The number of stops light reduction is equal to the base 2 logarithm of the filter factor.

 

[Roger Hicks]

There are different standards for NDs but as far as I recall (I've mislaid my B+W book] B+W use log values so 0,3 = 1 stop (2x), 1.0 = 3 stop (8x) etc. I could however be misremembering completely.

Cheers,

Roger

 

[RayPA]

Anders,

I just ordered an ND from B&H. I got the information from the B&H site, by clicking on the product description. It is pretty informative and explanatory. I wanted two stops and used that information, and got what I wanted. It's not here in front of me, but I THINK I got an ND 6 4x.

 

[richard_l]

Here's another way to think about filter factors. A filter factor of 4x means that the filter will admit/pass 1/4 of the light. This is equivalent to stopping down 2 stops. (Since each stop cuts the light in half, 2 stops would admit only 1/2 x 1/2 = 1/4 of the light.) Likewise, stopping down 3 stops would admit 1/2 x 1/2 x 1/2 = 1/8 of the light, so this is equivalent to the effect of an 8x filter factor.

The filter factor is convenient when using a non-TTL lightmeter. For example, to compensate for a 4x filter, just divide the film speed by 4 and set the meter accordingly.
(E.g., if the film speed is 400, set the meter to 400/4 = 100.)

 

[Roger Hicks]

Dear Richard,

That's the problem. 2x, 4x, 8x etc are nice and simple. So are 2 stops (4x), 3 stops (8x), 4 stops (16x) etc. So are the log ranges: 0,3 = 1 stop = 2x, etc. The catch lies in knowing which system is in use.

Cheers,

Roger

 

[RayPA]

I agree. Confusing enough, for sure. Here is the link to BH's B&W ND .6 39mm filter, with their explanation. I need to commit this to memory somehow, because I walked into a photo store on Friday and bought a 52mm ND filter, hoping to get the same 2 stop reduction and found myself confronted with the 1/2 & 1/4 option. I bought the 1/2. :bang: Oh well... 🙂

 

[richard_l]

If the filter says "8x", then 8 would be the ordinary filter factor ("x" standing for "times" as in multiplication). It if says "3 stops", then it's self-explanatory. I haven't seen any filters with the log numbers, but probably something like 0.9 without further designation would be a logarithm. (Logarithms are pure numbers. They have no associated units/dimensions).

Anyhow (for those not familiar with this stuff), to get from ordinary filter factors to the logarithmic number, just take the common (base 10) logarithm of the filter factor: log 2 = 0.3, log4 = 0.6, log 8 = 0.9. To get the filter factor from the logarithm, take the antilog: 10^0.3 = 2, 10^0.6 = 4, 10^0.9 = 8.