Here's a few examples of long time exposures with the B+W .
The last photo has the color cast intact , it's not terrible .
If you do decide to go with the screw on filter, don't forget to purchase some stop-down rings so you can use it on other lenses !
Gorgeous shots. I feel inspired to use my ND filters more.
Do you need 10 stops to accomplish this? Can you make do with 6 stops ND?
I usually shoot 30 second or longer but to get the effect in the above photos it's probably overkill. Depending on the rate of movement of the subject ( in this case water.) you could get the same misty, foggy results at shorter exposures. Clouds on the other hand tend to move slower so take more time to capture movement.