Neutral density filter
Under neutral density filter ( ND filter, neutral density filters, mostly: gray filter ) is understood ducted in photography glass or plastic panes of optical quality, which are screwed or plugged in front of the lens of the photographic apparatus in order to achieve uniform darkening the image. Gray filter are also in the film camera and TV camera technology as well as in lighting technology in use. You are homogeneously colored neutral gray so the color reproduction is not distorted.
Applications
In photography, the reduction of the incident through the lens quantity may be required if at a given film speed, the recording light is too bright and the Abblendmöglichkeit the lens is not enough to avoid overexposure or desired aperture value or shutter speed can not be maintained. This can be the case when lift the object to be photographed by choosing an open cover in front of blurred background or a wiping or flowing effect is to be achieved by long-term exposure.
In order to approximately represent the motion of falling water in photographs, you need 1/60 sec exposure time or longer. Is now the recording light so bright and / or the film speed so high that despite the smallest possible aperture on the lens, this 1/60 sec can not be adjusted for correct exposure, a gray filter is used. In particular, interfere with architectural shots persons or vehicles that move in front of the building. Here, you can extend the exposure time so that the threshold of the film or sensor is below, and persons or vehicles in the picture are not visible through the use of neutral density filters.
Also in the film technology neutral density filter can be used. Since film cameras frame by frame work with constant exposure time in bright scenes often the Abblendmöglichkeit of the lenses is not enough to achieve balanced exposures. Some camcorders also have built-in filters that can be added to turn.
In the light gray filter technique are used to reduce the amount of light emitted when the dimming of the headlights not desired or possible, because this would change the color temperature of the light.
Also, a variable type, based on the Polfiltereffekt, with the elongation factor 3-8 is common. Two polarizing filters are rotated against each other. If both polarizing filters in parallel, so the least attenuation occurs. When exactly vertical position to each other, the light is ideally completely blocked. When combining two linear polarization filters can be expected with a residual polarization, the auto-focus and exposure metering in cameras with beam splitters can affect (especially SLR ).
Characteristics
Standard types of gray filters have the extension factor of 2, 4 or 8, the filter mounts the designation " ND 2X ", " 4X ND " or " ND 8X " is then specified.
With stronger NDX Filter (Neutral Density ) or ODx (optical density) x is specified with a trailing number. When the large "X" missing and the factor name is indicated by the decimal point, for example, " ND 3.0 ", then this indicates to the logarithmic scale of damping according to the following formula:
The strength of the attenuation calculated from the logarithm of the ratio of the incident intensity and the measurable after the filter intensity.
Comparison Chart
The following table shows the common names of NDX (logarithmic scale), the indication of ND ... X ( x - fold prolongation of the closure time, linear scale) and the number of stops around which the amount of light decreases, dar. ND 0 corresponds to the waiver of a gray filter ( no filter effect ), polarizing filter lying around in their effect to ND 0.45 ND 8.0 and the indication represents a weakening of the intensity by a factor of 100 000 000 = 108
Unfortunately, the information on the ND are given different, ie most commercially offered ND8 Filter 0.9 ND filter. Here, the only ever to read the complete item description.