Neutral-density filter

In photography and optics, a neutral-density filter, or ND filter, is a filter that reduces or modifies the intensity of all wavelengths, or colors, of light equally, giving no changes in hue of color rendition. It can be a colorless (clear) or grey filter. The purpose of a standard photographic neutral-density filter is to reduce the amount of light entering the lens. Doing so allows the photographer to select combinations of aperture, exposure time and sensor sensitivity that would otherwise produce overexposed pictures. This is done to achieve effects such as a shallower depth of field or motion blur of a subject in a wider range of situations and atmospheric conditions.

For example, one might wish to photograph a waterfall at a slow shutter speed to create a deliberate motion-blur effect. The photographer might determine that to obtain the desired effect, a shutter speed of ten seconds was needed. On a very bright day, there might be so much light that even at minimal film speed and a minimal aperture, the ten-second shutter speed would let in too much light, and the photo would be overexposed. In this situation, applying an appropriate neutral-density filter is the equivalent of stopping down one or more additional stops, allowing the slower shutter speed and the desired motion-blur effect.

Neutral Density

The neutral density ( ) is a variable used in oceanography, introduced in 1997 by David R. Jackett and Trevor J. McDougall. It is function of the three state variables (salinity, temperature, and pressure) and the geographical location (longitude and latitude) and it has the typical units of density (M/V). The level surfaces of form the “neutral density surfaces”, which are the most natural layer interfaces stratifying the deep ocean circulation, along which the strong lateral mixing in the ocean occurs. These surfaces are used in the analyses of ocean data and to perform models of the ocean circulation. The formation of neutral density surfaces from a given hydrographic observation requires only a call to a computational code (available for Matlab and Fortran), that contains the computational algorithm developed by Jackett and McDougall.

Mathematical expression

A neutral density surface is the surface along which a given water mass will move, remaining neutrally buoyant.