For fun I like to play around with longer exposures. The first image (above) is a 30 second exposure. You can do this on a bright sunny day by using a filter on your lens or at night without a filter.
10 Stop Solid ND Filter - I used a BW 10 stop filter to get the top image. They are around $50-$100.
Tripod - Ideally a tripod is recommended. I actually forgot my tripod so set my camera up on a nearby pylon and used my lens hood to keep the camera upright and steady.
Focus First - once your camera is setup, focus the lens on the scene, then switch the lens to Manual Focus (MF). Then attach the filter. You cannot focus with the filter on because it is so dense you cannot see anything.
The light difference between the sky and the foreground can be greatly different. This means you will often get a correctly exposed foreground but there won't be any detail in the sky. I have two tips that I use that might be helpful for getting a better exposure.
1. Graduated …