In photography and optics, a neutral density filter or ND filter is a filter that reduces
or modifies the intensity of all wavelengths or colors of light equally, giving no changes
in hue of color rendition.
It can be a colorless or grey filter.
The purpose of a standard photographic neutral density filter is to reduce the amount of
light entering the lens.
Doing so allows the photographer to select combinations of aperture, exposure time and
sensor sensitivity which would otherwise produce overexposed pictures.
This is done to achieve effects such as a shallower depth of field and/or motion blur
of a subject in a wider range of situations and atmospheric conditions.
For example, one might wish to photograph a waterfall at a slow shutter speed to create
a deliberate motion blur effect.
The photographer might determine that to obtain the desired effect a shutter speed of ten
seconds was needed.
On a very bright day, there might be so much light that even at minimum film speed and
a minimum aperture, the ten second shutter speed would let in too much light and the
photo would be overexposed.
In this situation, applying an appropriate neutral density filter is the equivalent of
stopping down one or more additional stops, allowing for the slower shutter speed and
the desired motion-blur effect.
Mechanism For an 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 after the filter to the incident intensity,
shown as the following: Fractional Transmittance = 10-d, or
Uses
The use of an ND filter allows the photographer to use a larger aperture that is at or below
the diffraction limit, which varies depending on the size of the sensory medium 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.
ND filters can also be used to reduce the depth of field of an image where otherwise
not possible due to a maximum shutter speed limit.
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
and the aperture set as needed).
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.
Reducing depth of field in very bright light.
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
Extended time exposures Neutral density filters are used to control
exposure with photographic catadioptric lenses, since the use of a traditional iris diaphragm
increases 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.
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.
Large telescopes can cause the moon and planets to become too bright and lose contrast.
A neutral density filter can increase the contrast and cut down the brightness, making
the moon easier to view.
Varieties 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.
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.
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 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.
An inexpensive, homemade alternative to professional ND filters can be made from a piece of welder's
glass.
Depending on the rating of the welder's glass, this can have the effect of a 10-stop filter.
Specialist neutral density filters The two most common are the variable ND filters
and the extreme ND filters such as the Lee Big Stopper.
Variable neutral density filter The main disadvantage of neutral density filters
is that to be entirely flexible in your shooting you need to carry a range of different NDs.
This can become an expensive proposition, especially if using screw filters with different
lens filter sizes which would require carrying a set for each diameter of lens carried.
To counter this some manufacturers have created variable ND filters.
These work by placing two polarizing filters together at least one of which can rotate.
The rear polarizing filter cuts out light in one plane.
As the front element is rotated, it cuts out an increasing amount of the remaining light
the closer the front filters comes to being perpendicular to the rear filter.
By using this technique the amount of light reaching the sensor can be varied with almost
infinite control.
The advantages to this are that you get multiple ND filters in one package, the disadvantage
is a loss of image quality caused by both using two elements together and by combining
two polarizing filters.
Extreme ND filters To create ethereal looking landscapes and
seascapes with extremely blurred water, or other motion, has needed the use of multiple
stacked ND filters.
This had, as in the case of variable NDs, the effect of reducing image quality.
To counter this, some manufacturers have produced high quality, extreme ND filters.
Typically these are rated at a 10 stop reduction, allowing for very slow shutter speeds even
in relatively bright conditions.
ND filter ratings In photography, ND filters are quantified
by their optical density or equivalently their f-stop reduction.
In microscopy, the 'transmittance' value is sometimes used.
Note: Hoya, B+W, Cokin use code ND2 or ND2x, etc; Lee, Tiffen use code 0.3ND, etc; Leica
uses code 1x, 4x, 8x, etc See also
Graduated neutral density filter References
External links Neutral Density Filter Calculation Chart
ND Filters: Everything You Need to Know Neutral Density Filters and Graduated ND Filters
Neutral Density Filters: What are they & when to use them ?
Neutral Density Filter FAQ at Digital Grin Photography Forum