Neutral density filter

From Wikipedia, the free encyclopedia

Neutral Density Filters are often used to achieve motion blur effects with slow shutter speeds

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 ND filter with optical density d the amount of optical power transmitted through the filter is given by:

Fractional Transmittance = 10^-d

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 motion blurred effect. An ND filter can also be used to create motion blur, if the photographer closes the aperture to a diameter such as f/32, as extremely small apertures tend to require slower shutter speeds. They also soften the image due to diffraction.

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. With using ND filters to limit the light instead of the aperture, the photographer can then set the shutter speed according to the particular motion desired (blur of water movement, for example) and the aperture set for maximum sharpness. Using a digital camera, which provides results right away, a photographer could find the right 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.

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 consists 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 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.

ND filters are quantified by their optical density or equivalently their f-Stop reduction as follows:

Another practical way of determining what type of ND filter to use is by the percent of light that the filter allows to pass (transmittance). This parameter is typically applied to microscopy applications versus photography applications. Here is a more complete list of Filter Optical Density versus percent light transmitted: [1]

[edit] External links

Neutral Density Filters: What are they & when to use them ?