Neutral Density Filters in a nutshell serve as sunglasses on your lens to reduce the amount of light entering your lens and thereby allow you to shoot with extended exposure times. You may want to do this, for instance, if you would like to create a motion-blur with a slow shutter speed as with the image above, giving you that creamy, soft texture of the sea.
These filters can be purchased individually or in combinations with varying opaque density. I am working with a three pack of filters by Tiffen. You can use an individual filter which screws over your lens, or stack them to provide additional opacity. There is a lot written on the subject and I won’t delve into that in this post, but two tips come readily to mind. You MUST use your tripod and you should experiment with varying shutter speeds, aperture settings and sensor sensitivity.
Rich Smukler specializes in Landscape and Fine-Art Photography from his studio in South Florida. His works have been featured in numerous museums, galleries and private collections internationally. You can see more of his works at http://www.richsmuklerphoto.com. (Kick back and stay awhile).