Nick, You sure about that?Originally Posted byNick Zentena

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.