ND Filter

ND Filter

Neutral Density Filter

In photography and optics, the neutral density filter or ND filter can be constructed from colorless (clear) or grey filter. The ideal neutral density filter can reduce and/or modify the intensity of all the wavelengths or colors of light equally, which results in no changes to the hue of color rendition. The main purpose of using standard photographic neutral density filters is to give the photographer much greater flexibility to vary the aperture, exposure time and/or motion blur of the subject in atmospheric conditions and different situations.

Uses of ND Filter

The use of an ND filter gives the photographer the ability to utilize a much larger aperture that is at or below the diffraction limit, which will vary depending on the size of the sensor and for most cameras, it is between f/8 and f/11. Aside from reducing the aperture to limit the amount of light, the photographer can also add an ND filter to limit the amount of light, and he can then set the shutter speed depending on the particular motion desired (blur of moving water, for example) and the aperture can be set as needed (small aperture to maximize sharpness or large aperture to enable narrow depth of field (subject is in focus and the background out of focus).

If the photographer is using a digital camera, he can see the images instantly, and he can also choose the most appropriate ND filter to use for the scene currently being photographed by first knowing the correct aperture to use for maximum sharpness. The shutter speed can be selected by looking for the desired blur from the movement of the subject.

The digital camera would be set up for this using manual mode, and then the overall exposure will be adjusted darker by changing either the aperture or shutter speed, taking note of the number of stops required to bring the exposure to that which is desired.

The required offset would then be the amount of f-stop needed in the Neutral Density filter to use for that scene.

Examples of this use include:

• Blurring the motion of water (e.g. waterfalls, seas, canals, streams, rivers, oceans).

• Decreasing the depth of field in areas or instances with very bright light (e.g. daylight).

• If you are using a flash on a camera with a focal-plane shutter, the exposure time is generally limited to the maximum speed –oftentimes, it is 1/250th of a second, at best – at which the whole film or sensor is exposed to light in one instance. Without using an ND filter, this will require the use f8 or higher.

• Using a larger aperture in order to stay away from the diffraction limit.

• Reduce the visibility of objects that are moving

• Add motion blur to photographic subjects

Neutral density filters are also used to control the exposure with photographic catadioptric lenses, since using traditional iris diaphragm will increase the ratio of the central obstruction that can be found in those systems which can lead to poor performance.

ND filters are also used in several high-precision laser experiments simply because the power of the laser cannot be adjusted without varying other properties of the laser light. Aside from that, most lasers have a minimum power level at which they can be operated. To achieve the required light attenuation, several neutral density filters can be placed in the path of the laser beam.

Varieties of ND Filter

A graduated ND filter is the same except for the fact that the intensity varies across the surface of the filter. This is very useful when one region of the image is bright and the rest of the image is not, for example: in the picture of a sunset.

The transition area of the ND Filter, or edge, is usually available in different variations (soft, hard, attenuator). The most common type is the soft edge and it can provide a smooth transition from the ND side and the clear side of the filter. Hard edge grads have a very sharp transition from ND to clear and the attenuator edge will change gradually over most of the filter so the transition is less noticeable.

Another kind of ND filter configuration is the ND Filter-wheel. This is composed of two perforated glass disks which have possess progressively denser coating that is applied around the perforation on the face of each disk. Whenever the two disks are counter-rotated in front of each other they would gradually and evenly went from 100% transmission to 0% transmission. They are usually used on catadioptric telescopes and in any optical system that is needed to work at 100% of its aperture

Practical ND filters aren’t perfect, because they do not reduce the intensity of all the light wavelengths equally. Sometimes, this will create color casts in recorded photos, particularly with cheap filters. More importantly, most ND filters are only specified to work over the visible region of the spectrum, and they do not proportionally block all the wavelengths of ultraviolet or infrared radiation. This might be dangerous if you are using ND filters to view sources (example: sun or white-hot metal or white-hot glass) emitting intense invisible radiation, since your eye can be damaged even though the light source does not even look bright when they are viewed through the filter. Special filters should be used if such sources are to be viewed safely.

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

	lens area opening, as fraction of the complete lens	Filter 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%