Over the holidays I picked up a Lee Super Stopper neutral density filter. I have been using neutral density filters for a couple years now like the Lee Big Stopper and Little Stopper. The Little Stopper is 6 stops, the Big Stopper is 10 Stops and the Super Stopper is 15 stops.
When it comes to Stops, a single stop is a doubling or halving of your shutter speed, aperture, or ISO. The Little Stopper being 6 stops, multiply your shutter speed by 64. For example, if your unfiltered shutter speed is 1/50 sec. your new shutter speed would be 1.3 secs.
The Big Stopper will multiply your shutter speed by roughly 1024 seconds. The big Stopper is 4 stops above the Little Stopper so if you start with 64, double that to 128, double that to 256, then 512, then 1024. Using the same shutter speed of 1/50 sec. you new shutter speed will be 20 sec.
Now I get my Super Stopper and the shutter speed is multiplied by 32,000. The same shutter speed of 1/50 sec. now becomes 640 secs or 10 min 40 sec. With that long of a shutter, the clouds lose all textures and shapes, water becomes completely smooth. The one advantage is I can now take long exposure photos in the middle of a sunny day using a 2 or 3 minute exposure. This will open up more fine art photos and I can turn ordinary daytime photos into long exposure black and whites.
The downside to the Lee Super Stopper is the same as the Big Stopper, it has a blue cast and is slightly darker than 15 stops. I use the Sony A7Rii camera and can manually adjust the white balance so I turn it almost to 10000 kelvin to remove the blue cast. I also leave the shutter open for about 30 seconds longer than I should to correct for the extra darkness. This will be something to test before you actually need to use the filter.
Here are some example photos I took. The first one is no filter to set my correct exposure at 1/50 sec. The second is using the Super Stopper at 687 seconds. Remember to always cover your camera with long exposures to protect against light leak. You can see in the third photo the streak of light in the corner. I did not have my camera covered do to high wind and my microfiber cloth kept blowing off. This filter is a lot of fun to use a worth purchasing if like to have fun with your photography.