An ND filter, or neutral-density filter, is a filter used in photography and optics that reduces or modifies the intensity of all wavelengths, or colors, of light equally, giving no changes in hue of color rendition. It acts like sunglasses for your camera lens, blocking light coming into the lens without changing the color of the light. The use of an ND filter allows the photographer to use a larger aperture that is at or below the diffraction limit, which varies depending on the size of the sensory medium (film or digital) and for many cameras is between f/8 and f/11, with smaller sensory medium sizes needing larger-sized apertures, and larger ones able to use smaller apertures.

ND filters are created in stops, which means they are halving or reducing the amount of light. For example, a 1-stop ND filter will be stopping the light by 50% or half, while a 10-stop filter is stopping the light by 10 halves in a row. ND filters can also be used to reduce the depth of field of an image (by allowing the use of a larger aperture) where otherwise not possible due to a maximal shutter speed limit.

There are two main types of ND filters: fixed ND filters and variable ND filters. Fixed ND filters have a predetermined density, while variable ND filters are essentially two polarizers placed together with one plane preventing a fixed amount of light and the other rotating to prevent incremental amounts of light from entering the camera. Variable ND filters can be advantageous to the photographer on the go, as they can be rotated to get the desired effect without having to change filters.

The downsides to most ND filters are unpredictable “color shift” and/or vignetting. Inexpensive filters can sometimes create color casts in recorded images, particularly with inexpensive filters. Some popular brands of ND filters include HiTech, Lee, Nisi, and Cokin.