A neutral density filter placed over the lens of a camera can help reduce the amount of light entering the camera. There are times when a photographer wants a slow shutter speed, to show motion, rather than freeze it.  A slow shutter speed in the bright light of a clear sunny day is not easy to accomplish, if at all possible, without a neutral density filter, a piece of dark dense glass which does not effect color, but does restrict the amount of light able to pass through that dense filter. To get a proper exposure, the shutter needs to remain open longer for a correct exposure. 

Using a slow shutter speed of about 1 second on a bright sunny day would generate significant over exposure, unless a neutral density (ND) filter is employed. There are other variables that contribute to correct exposure such as the size of the aperture, wide open or closed down, (f/)and the light sensitivity of the ‘film’ sensor (ISO).  An ND filter is one way to have greater control over the variables.

ND filters are gauged in terms of ‘stops.’ The more stops, the less light is allowed into the camera through the lens. The densest of filters is ’10 stops.’ Such a filter allowed me to capture the following images, on a bright sunny day at an exposure of 1 second (with a small aperture of f/11 and a relatively low sensitivity of ISO 200). This duration of exposure allowed the camera to capture the water breaking over the wall such that it now appears as silky, milky and in motion, rather than frozen.

If you are wanting to experiment with this kind of photography, purchase the best quality 10 stop ND filter that fits onto the largest lens you have. To use this one filter on any lens you have, purchase step down rings so the larger filter can fit onto a smaller lens filter size. All you need is a camera in which you can adjust shutter speed, aperture and light sensitivity, a lens, an ND filter and, with long exposures, a tripod is a necessity.