ND FILTER

 

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 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 purpose of standard photographic neutral density filters is to allow the photographer greater flexibility to change the aperture, exposure time and/or blur of subject in different situations and atmospheric conditions.

For a ND filter with optical density d the amount of optical power transmitted through the filter, which can be calculated from the logarithm of the ratio of the measurable intensity (I) after the filter to the incident intensity (I₀), shown as the following:

Fractional Transmittance (^I⁄_I0) = 10^-d, or

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 more pleasing effect.

Comparison of two pictures showing the result of using a ND-filter at a landscape. The first one uses only a polarizer and the second one a pol and a 1000x ND-Filter (ND3.0).

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 diaphragmincreases 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.

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.

The transition area, or edge, is available in different variations (soft, hard, attenuator). The most common is a soft edge and provides a smooth transition from the ND side and the clear side. Hard edge grads have a sharp transition from ND to clear and the attenuator edge changes gradually over most of the filter so the transition is less noticeable.

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 are quantified by their optical density 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%

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.

DIGITAL CAMERAS

ENG cameras

Sony camera head with Betacam SP dock recorder.

Though by definition, ENG (Electronic News Gathering) video cameras were originally designed for use by news camera operators, these have become the dominant style of professional video camera for most productions, from dramas to documentaries, from music videos to corporate training. While they have some similarities to the smaller consumer camcorder, they differ in several regards:

• ENG cameras are larger and heavier, and usually supported by a shoulder stock on the cameraman's shoulder, taking the weight off the hand, which is freed to operate the lens zoom control. The weight of the cameras also helps dampen small movements.
• 3 CCDs are used instead of one, one for each primary color
• They have interchangeable lenses.
• All settings, white balance, focus, and iris can be manually adjusted, and automatics can be completely disabled.
• The lens is focused manually and directly, without intermediate servo controls. However the lens zoom and focus can be operated with remote controls in a studio configuration.
• Professional BNC connectors for video and at least two XLR input connectors for audio are included.
• A complete time code section is available, allowing time code presets; and multiple cameras can be timecode-synchronized with a cable.
• "Bars and tone" are available in-camera (the color bars are SMPTE (Society of Motion Picture and Television Engineers) Bars, a reference signal that simplifies calibration of monitors and setting levels when duplicating and transmitting the picture. )
• Recording is to a professional medium like some variant of Betacam or DVCPRO or Direct to disk recording or flash memory. If as in the latter two, it's a data recording, much higher data rates (or less compression) are used than in consumer devices.
• The camera is mounted on tripods and other supports with a quick release plate.
• A rotating behind-the-lens filter wheel, for selecting an 85A and neutral density filters.
• Controls that need quick access are on hard physical switches, not in menu selections.
• Gain Select, White/Black balance, color bar select, and record start controls are all in the same general place on the camera, irrespective of the camera manufacturer.
• Audio is adjusted manually, with easily accessed physical knobs.

EFP Camera operator at a baseball game.

EFP Cameras

Electronic Field Production cameras are similar to studio cameras in that they are used primarily in multiple camera switched configurations, butoutside the studio environment, for concerts, sports and live news coverage of special events. These versatile cameras can be carried on the shoulder, or mounted on camera pedestals and cranes, with the large, very long focal length zoom lenses made for studio camera mounting. These cameras have no recording ability on their own, and transmit their signals back to the broadcast truck through a triax, fibre optic or the virtually obsolete multicore cable.

Dock cameras

Some manufacturers build camera heads, which only contain the optical block, the CCD sensors and the video encoder, and can be used with astudio adapter for connection to a CCU in EFP mode, or various dock recorders for direct recording in the preferred format, making them very versatile. However, this versatility leads to greater size and weight. They are favored for EFP and low-budget studio use, because they tend to be smaller, lighter, and less expensive than most studio cameras.

A remote-controlled camera mounted on a miniature cable car for mobility.

Remote cameras

Remote cameras are typically very small camera heads designed to be operated by remote control. Despite their small size, they are often capable of performance close to that of the larger ENG and EFP types.

"Lipstick cameras" are so called because the lens and sensor block combined are similar in size and appearance to a lipstick container. These are either hard mounted in a small location, such as a race car, or on the end of a boom pole. The sensor block and lens are separated from the rest of the camera electronics by a long thin multi conductor cable. The camera settings are manipulated from this box, while the lens settings are normally set when the camera is mounted in place.

Block cameras are so called because the camera head is a small block, often smaller than the lens itself. Some block cameras are completely self contained, while others only contain the sensor block and its pre-amps, thus requiring connection to a separate camera control unit in order to operate. All the functions of the camera can be controlled from a distance, and often there is a facility for controlling the lens focus and zoom as well. These cameras are mounted on pan and tilt heads, and may be placed in a stationary position, such as atop a pole or tower, in a corner of a broadcast booth, or behind a basketball hoop. They can also be placed on robotic dollies, at the end of camera booms and cranes, or "flown" in a cable supported harness, as shown in the illustration.