Neutral density filters are one of the most popular filters for digital camera lenses. They are able to reduce the amount of light entering the camera, which allows for a longer exposure time. The longer exposure time will emphasize motion, making the scene seem almost surreal. The surreal effect is often witnessed on photographs of moving water, like rivers and waterfalls. The filter can create tempestuous water and blurred motion –whether the subject is people, moving cars, or blowing grass.
Other effects are a shallower depth of field and a sharper image, both ideal qualities. While the filter is not as commonly used for these applications, they are still beneficial. The filter works well in very bright light to enable a shallow depth of field, resulting in background blur and isolation of the subject.
How does the filter work? It is actually just a piece of semi-transparent glass that inhibits a controlled fraction of incoming light uniformly, meaning it does not alter the image contrast or sharpness. Because it's equal across the visible spectrum, it also does not introduce color cast, which gives the filter its name neutral. The filters are categorized by the strength of their light-reducing ability. Stronger filters are darker shades of gray.