In photography and optics, a neutral density filter or ND filter can be a colorless (clear) or grey filter. An ideal neutral density filter reduces and/or modifies intensity of all wavelengths or colors of light equally, giving no changes in hue of color rendition.
The use of an ND filter allows the photographer to utilize 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.
Instead of reducing the aperture to limit light, the photographer can add a ND filter to limit light, and can then set the shutter speed according to the particular motion desired (blur of water movement, for example) and the aperture set as needed (small aperture for maximum sharpness or large aperture for narrow depth of field (subject in focus and background out of focus). Using a digital camera, the photographer can see the image right away, and can choose the best ND filter to use for the scene being captured by first knowing the best aperture to use for maximum sharpness desired. The shutter speed would be selected by finding the desired blur from subject movement. The camera would be set up for these in manual mode, and then the overall exposure then adjusted darker by adjusting either aperture or shutter speed, noting the number of stops needed to bring the exposure to that which is desired. That offset would then be the amount of stop needed in the ND filter to use for that scene.
Examples of this use include:
- Blurring water motion (e.g. waterfalls, rivers, oceans).
- Reducing depth of field in very bright light (e.g. daylight).
- When using a flash on a camera with a focal-plane shutter, exposure time is limited to the maximum speed -often 1/250th of a second, at best- at which the entire film or sensor is exposed to light at one instant. Without an ND filter this can result in the need to use f8 or higher.
- Using a wider aperture to stay below the diffraction limit.
- Reduce the visibility of moving objects
- Add motion blur to subjects
Neutral density filters are used to control exposure with photographic catadioptric lenses, since the use of a traditional iris diaphragm increases the ratio of the central obstruction found in those systems leading to poor performance.
ND filters find applications in several high-precision laser experiments because the power of a laser cannot be adjusted without changing other properties of the laser light (e.g. collimation 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.
Large telescopes can cause the moon and planets to become too bright and lose contrast. A neutral density filter can increase the contrast and cut down the brightness, making the moon easier to view.
ND filter types
In photography, ND filters are quantified by their optical density or equivalently their f-stop reduction. In microscopy, the 'transmittance' value is sometimes used.lens area opening, as fraction of the complete lens | optical density | f-stop reduction | % transmittance | |
|---|---|---|---|---|
| 1 | 0.0 | 100% | ||
| ND2 | 1/2 | 0.3 | 1 | 50% |
| ND4 | 1/4 | 0.6 | 2 | 25% |
| ND8 | 1/8 | 0.9 | 3 | 12.5% |
| ND16 | 1/16 | 1.2 | 4 | 6.25% |
| ND32 | 1/32 | 1.5 | 5 | 3.125% |
| ND64 | 1/64 | 1.8 | 6 | 1.563% |
| ND128 | 1/128 | 2.1 | 7 | 0.781% |
| ND256 | 1/256 | 2.4 | 8 | 0.391% |
| ND512 | 1/512 | 2.7 | 9 | 0.195% |
| ND1024 | 1/1024 | 3.0 | 10 | 0.098% |
| ND2048 | 1/2048 | 3.3 | 11 | 0.049% |
| ND4096 | 1/4096 | 3.6 | 12 | 0.024% |
| ND8192 | 1/8192 | 3.9 | 13 | 0.012% |
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