Neutral density filter
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In photography and optics, a neutral density filter or ND filter is a "grey" filter. An ideal neutral density filter reduces light of all wavelengths or colors equally. The purpose of standard photographic neutral density filters is to allow the photographer greater flexibility to change the aperture or exposure time, allowing for more control, particularly in extreme circumstances.
For a transmissive ND filter the amount of optical power transmitted throught the filter is given by:
Fractional Transmittance = 2-ND = 10-OD
where OD = optical density.
For example, on a very bright day, one might wish to photograph a waterfall at a slow shutter speed to create a deliberate motion blur effect. In order to do this, one would need a shutter speed on the order of tenths of a second. There might be so much light that even at minimum film speed and a minimum aperture such as f/32, the corresponding shutter speed would still be too fast. In this situation, by applying an appropriate neutral density filter one or more stops can be taken out of the exposure, allowing a slow shutter speed and more pleasing effect. Alternatively one can use a wider aperture to narrow the plane of focus and make the subject stand out from a blurred background.
Another use of neutral density filters is in controlling exposure with mirror-lenses catadioptric optics, since the use of a traditional iris diaphragm increases the ratio of the central obstruction found in those systems leading to poor performance.
A graduated ND filter is similar except the intensity varies across the surface of the filter. This is useful when one region of the image is bright and the rest is not, as in a picture of a sunset.
Another type of ND filter configuration is the ND Filter-wheel. It consist of two perforated glass disks which have progressively denser coating applied around the perforation on the face of each disk. When the two disks are counter-rotated in front of each other they gradually and evenly go from 100% transmission to 0% transmission. These are used on catadioptric telescopes mentioned above and in any system that is required to work at 100% of its aperture (usually because the system is required to work at its maximum angular resolution).
Practical ND filters are not perfect as they do not reduce the intensity of all wavelengths equally. This can sometimes create color casts in recorded images, particularly with inexpensive filters. More significantly, most ND filters are only specified over the visible region of the spectrum, and do not proportionally block all wavelengths of ultraviolet or infrared radiation. This can be dangerous if using ND filters to view sources (such as the sun or white-hot metal or glass) which emit intense non-visible radiation, since the eye may be damaged even though the source does not look bright when viewed through the filter. Special filters must be used if such sources are to be safely viewed.
ND filters find applications in several high-precision laser experiments. This is because the power of a laser cannot be adjusted without changing other properties of the laser light (e.g parallelicity of the beam). Moreover, most lasers have a minimum power setting at which they can be operated. To achieve the desired light attenuation, one or more neutral density filters can be placed in the path of the beam.
ND filters are quantified by their optical density and in photographic terms produce f-Stop reduction as follows:
| Filter Factor (NDx) | Filter Optical Density | f-Stop Reduction |
|---|---|---|
| 2 | 0.3 | 1 |
| 4 | 0.6 | 2 |
| 8 | 0.9 | 3 |
| 64 | 1.8 | 6 |
| 1,000 | 3.0 | 10 |
| 10,000 | 4.0 | 13 |
| 1,000,000 | 6.0 | 20 |
