neutraldensity

Neutral density (ND) refers to optical filters or materials that reduce light entering a camera without altering its color balance. In photography and filmmaking, an ND filter attenuates all wavelengths equally, producing a darker image.

By lowering the amount of light, ND filters allow longer exposures or wider apertures in bright scenes. This enables motion blur for moving water or clouds, and shallower depth of field in daylight. They are typically made of glass or resin and often include broadband coatings to minimize reflections.

There are fixed-density ND filters and variable ND filters. Fixed-density filters have a constant attenuation factor, such as ND2, ND4, or ND8, while variable ND uses two polarizing layers to adjust transmission and can produce a wide range of densities.

Measurement and labeling: optical density (OD) and filter factors. The stops of light reduced equal log2 of the inverse transmission. For example, ND2 transmits 50% (1 stop); ND4 transmits 25% (2 stops). Some manufacturers label ND0.3, ND0.6, ND0.9, corresponding to 1, 2, and 3 stops.

Common uses include landscape and long-exposure photography, where motion of water, clouds, or traffic is desired; cinema and video to maintain exposure levels with moving cameras and to keep aperture control. Color neutrality and optical quality are important considerations.

Limitations: higher-density filters can introduce color casts, require careful cleaning and mounting, and may cause vignetting on wide-angle lenses. Proper use and testing help preserve image color fidelity and sharpness.