Neutral density filter

A neutral density filter with a shutter speed extension factor of 64

An ND 0.6 filter (4X) extends the shutter speed by two f-stops.

Neutral density filters are mostly used to achieve motion blur despite high light intensity.

In photography , neutral density filters ( ND filters , neutral filters , mostly gray filters ) are glass or plastic panes of optical quality that are screwed or inserted in front of the lens of the camera in order to achieve even darkening in the image. Neutral density filters are also used in film and television camera technology as well as in lighting technology. They are colored homogeneously neutral gray so that the color rendering is not falsified. In practice, however, you will often find different color casts from different manufacturers that have to be removed in post-processing.

Special neutral filters, solar filters , are used for solar observation , which, however, are only allowed to let through less than 0.1 per thousand of solar radiation .

Applications

Comparison of two pictures. The first without and the second with a 1000X ND filter. The actual filter factor is not 1000, but approx. 1666.

In photography, it may be necessary to reduce the amount of light entering through the lens if, with a given film sensitivity, the exposure light is too bright and the lens cannot be dimmed to avoid overexposure or if the desired aperture values or exposure times cannot be adhered to. This can be the case if the object to be photographed is to stand out against a blurred background by choosing an open aperture or if a wiping or flowing effect is to be achieved through long-term exposure .

In order to be able to represent the movement of falling water photographically, one needs 1/60 sec. Exposure time or longer. If the exposure light is so bright and / or the film sensitivity is so high that this 1/60 second cannot be set for correct exposure despite the smallest possible aperture on the lens, a neutral density filter is used. People or vehicles moving in front of the building are particularly annoying when taking architectural photos. By using gray filters, the exposure time can be extended so that the stimulus threshold of the film or image sensor is not reached and people or vehicles are not visible in the image.

Neutral density filters are also used in film technology. Since a constant exposure time (usually - depending on the frame rate - 1/48, 1/50 or 1/60 sec.) Is required when recording films or videos, the option of dimming the lenses is often not sufficient for bright subjects. to achieve well-exposed images. Too much dimming would lead to a reduction in sharpness due to the diffraction blurring that occurs, while a shortening of the exposure time would lead to unnatural, stroboscopic movements. A neutral density filter is therefore required in such situations. Professional video cameras often have built-in neutral density filters that can be switched on in several stages if necessary.

In lighting technology, neutral density filters are used to reduce the amount of light emitted if dimming the headlights is not desired or possible, for example if this would change the color temperature of the light.

Variable ND filters

In addition to conventional gray filters with a fixed strength, there are also variable filters (also known as ND faders ). Technically, this involves two polarization filters arranged one behind the other , which are rotated against each other, which changes the light transmission. If both polarizing filters are parallel, the slightest attenuation occurs. In theory, if they are positioned exactly perpendicular to each other, the light is completely blocked. Variable gray filters are particularly popular for video recordings with single-lens reflex cameras or system cameras because, unlike many video cameras, these do not have a built-in gray filter, and changing the filters frequently when the light situation changes is time-consuming or, in some situations, hardly possible.

However, variable neutral density filters have some disadvantages compared to conventional ones with a fixed strength:

With increasing intensity, the darkening becomes uneven, and lighter and darker areas can arise in the image. In extreme cases, if the strength of the filter is set too high, this ultimately leads to a black, cross-shaped pattern appearing over the entire image. The shorter the focal length of the lens, the stronger the effect. The practically usable range of variable neutral density filters is limited by this effect to about 2 to 32 times, whereby the range can also be shifted upwards through the combination with a conventional neutral density filter. The usable area is usually marked by min / max markings and exceeding this range is sometimes mechanically blocked, but many manufacturers tend to design the supposedly usable area very generously, so that disruptive effects can occur even before the specified maximum position is reached.

Since these are polarization filters , they also cause a polarization filter effect, which can be undesirable in some situations, as this can greatly change the natural colors and contrasts (for example, the sky can appear unnaturally dark in unfavorable situations). With some high-quality variable gray filters, in addition to the front filter element, the entire filter is also designed to be rotatable in order to be able to minimize unwanted polarizing filter effects.

There may be a visible loss of sharpness, especially with inexpensive, variable gray filters of poor quality.

The exact, currently set strength of the filter cannot be determined. This is relevant if the required exposure time and aperture are to be calculated, for example for long exposures. With variable gray filters, correct exposure can therefore only be achieved by trial and error or by means of visual control.

When combining two linear polarization filters, a residual polarization must be expected, which can affect the autofocus and exposure metering in cameras with beam splitters (especially single lens reflex cameras ).

These disadvantages can be reduced to a certain extent by the use of high-quality variable neutral density filters, but cannot be completely avoided.

Markings

Standard types of neutral density filters have an extension factor of 2, 4 or 8. The designation ND 2X , ND 4X or ND 8X is indicated on the filter holder .

For stronger filters, NDx (neutral density) or ODx (optical density) is indicated with a number x after it. If the capital X designation is missing and the factor is given with a decimal point, for example for ND 3.0 , this indicates the logarithmic attenuation scale using the following calculation formula:

{\ displaystyle x = \ log {\ frac {I_ {0}} {I}}}

The strength of the attenuation is calculated from the decadic logarithm of the quotient of the incident intensity I ₀ and the intensity I that can be measured after the filter .

Comparison table

The following table shows the usual designations of NDx (logarithmic scale), the specification of ND… X (x-fold increase in shutter speed, linear scale) and the number of f-stops by which the amount of light is reduced. ND 0 corresponds to this No neutral density filter (no filter effect), polarization filters have an effect of around ND 0.45 and the specification ND 8.0 means a weakening of the intensity by a factor of 100,000,000 = 10 ⁸ .

The format in which the filter strength is specified varies depending on the provider. Many filters commercially available as ND8 actually have a neutral density of 0.9, so they are named based on their extension factor.

Neutral density ND, NDx	Permeability	Extension factor shutter speed or filter "ND ... X" -fold	Number of f-stops (catches of the cover)	Fraction of the original amount of light
0.0	100%	1	0.0	1
0.3	50%	2	1.0	1/2
0.45	35%	3	1.5	1/3
0.6	25%	4th	2.0	1/4
0.9	12.6%	8th	3.0	1/8
1.0	10.0%	10	3.3	1/10
1.2	6.3%	16	4.0	1/16
1.8	1.6%	64	6.0	1/64
2.0	1.0%	100	6.6	1/100
3.0	0.1%	1,000	10	1/1000
4.0	0.01%	10,000	13	1/10000
5.0	0.001%	100,000	17th	1/100000
6.0	0.0001%	1,000,000	20th	1/1000000
7.0	0.00001%	10,000,000	23	1/10000000
8.0	0.000001%	100,000,000	27	1/100000000

calibration

ND filters - especially those with a high filter strength - often do not correspond exactly to the specified density. In the case of ND filters with a neutral density of ND 3.0, deviations of half a f-stop to a full f-stop are within the usual range. It is therefore advisable to calibrate the filter strength before using the filter.

The image results with and without a filter are compared with the calculated exposure time and then the necessary exposure time when using the filter is adjusted until the results are congruent.

Calibration can also be performed using a smartphone app.

Color fastness

Many ND filters have a color cast because the longer exposure times allow more light to pass through in the infrared range. By using an infrared protective layer, this effect can be eliminated and the natural color restored.

literature

Rudolph Hanke, `` Filter-Faszination '', Monheim / Bayern 1979, pages 70 f., ISBN 3-88324-991-2

Individual evidence

↑ Cora Banek, Georg Banek: Learning to take photos . 1: The technical basics. dpunkt, 2010, ISBN 978-3-89864-648-2 , p. 160 .

Web links

Commons : Neutral density filter - collection of images, videos and audio files

[1] Cora Banek, Georg Banek: Learning to take photos . 1: The technical basics. dpunkt, 2010, ISBN 978-3-89864-648-2 , p. 160 .