Quote Originally Posted by Nick Zentena
Hoya ND2 are 1 stop. A filter factor of 2. B&W ND filter of 1 would be 3 1/3 stops if they made one. Every 0.3 is equal to one stop of light reduction. So check what the maker means.

The Cokins are supposed to be grey and not neutral. You'd want to avoid those.
Nick, You sure about that?

I believe a "Factor" of 2 would be 1.4 stops and a factor of 1 would be 1 stop. I don't know about Hoya but I have a little booklet from B+W that says their ND Filters:

Filter
101 2X factor = 1 Stop
102 4X factor = 2 Stop
103 8X factor = 3 Stop
106 64X factor = 6 Stop
110 1000X factor = 10 Stops

Seems their ND Filter # hold a clue to Stops. But this is not true of their Contrast Filters where the square root of the X factor determines the Stops.