I have a B+W 10-stop neutral density filter and tried it out last night on the waterfront.
As you can see, this filter causes heavy vignetting. Well, the whole point of a really dark filter like this is to slow down the shutter speed dramatically. The clouds moved during the exposure, so they’re a little blurred. The ripples in the water blurred to a frosty, frozen look. I’m not completely satisfied with this result, but I know the odds of perfecting something new the first time out are quite slim.
You can see the Washington State Capitol above the boats to the left. This photo was taken after sundown. Here the water blur is more evident, but the farther clouds didn’t blur as much as the first photo. I can’t describe it accurately, but light collected over half a minute looks different in a photo than light gathered in a fraction of a second. For comparison, here’s a photo with the same camera, lens, location, but no ND filter and a much shorter exposure.
Compare the water ripples and more defined clouds to the other two photos. But beyond the obvious, there’s just a whole different look to long exposure photos.