the green wavelengths of light. This selective trans-
mission is what causes the filter to appear green to
the eye.
Other filters remove light non-selectively by
wavelength. These filters, known as neutral density
filters, remove an equal portion of all wavelengths,
reducing the amount of light being transmitted
through the filter in general.
Other filters, called polarizers, remove light ac-
cording to its angle of polarization, or vibration.
These filters are used to reduce reflections and cap-
ture patterns of refraction as color.
Another method of categorization of filters is
through how transmission of light is accomplished.
Transmission filters absorb other wavelengths,
while generally transmitting a category of wave-
length through the filter. These filters can be plastic
or glass, and have the common distinction of being
the color that they transmit. Transmission filters
are the most common type of filters, since the
wavelength transmitted can remain fairly general,
i.e., green or ultraviolet radiation. They can be
made of gelatin, plastic, or glass, and come in
several sizes and shapes, from screw-mount filters
designed to attach to the front of a camera lens to
large acetate filters used to change the color of a
light source.
Other filters reflect the wavelengths of light not
transmitted. These filters, known as interference or
barrier filters, can be made to be much more selec-
tive in terms of the final transmitted wavelength.
For example, instead of transmitting all greens, an
interference filter can be made to transmit only from
400–410 nm. This specificity makes these filters an
ideal choice for scientific and medical applications
where more precise narrow-band transmission is
needed. Interference filters often are made of glass
and have a mirrored appearance. The color of the
wavelength band is only apparent on an interference
filter if it is held to a light source.
Special application filters include soft-focus fil-
ters, which scatter light across the filter, creating a
softened, pastel effect, and cross-screen filters,
which produce a ‘‘starburst’’ effect around lights.
Uses of Filters
By absorbing (or reflecting) certain wavelengths,
color filters can create contrast effects, particularly
on black and white film. A color filter will lighten
colors adjacent to its color on the color wheel, and
darken colors opposite, also known as complemen-
tary colors. If a green filter is used on a scene of
apples on a green background, for example, the
apple will appear darker in tone while the back-
ground will render lighter on black and white film.
This is due to the filter’s absorption of red light,
making the apples very dark, and the transmission
of green, lightening that part of the subject.
This method of using filters to enhance or change
contrast on black and white films can also be used
to overcome film’s inherent blue sensitivity. Black
and white films are more sensitive to colors in the
blue and ultraviolet wavelengths than the human
eye, and therefore, render these areas lighter on film
than we see them. As a result, images that include
sky often render as bright white, even if it’s a blue
sky on a cloudless day. A popular technique to
darken the sky on black and white film is to use
one of blue’s complements (yellow, orange, or red).
Color correction filters, more subtly colored
than contrast filters, are designed to correct film’s
response under certain light sources. For example,
a magenta-toned filter is used to correct for the
green cast that accompanies photography under
fluorescent illumination.
Neutral density filters, which remove light
equally from all wavelengths, are used in a variety
of applications. Most commonly, neutral density is
used when it is important to reduce the amount of
light striking the film without altering the color.
For example, if photographing a busy city street
at high noon, there may be too much light to use an
appropriately slow shutter speed to blur the action
of the fast-moving cars and pedestrians. A neutral
density filter could be used to reduce the amount of
light, thereby allowing the photographer to use an
exposure of several minutes if desired.
Polarizing filters remove light according to the
angle it enters the filter. The effect is one of redu-
cing reflections, increasing contrast and saturation,
and darkening skies. Since much of these factors
involve the scattering of light, polarizing filters ef-
fectively remove the glare created.
Filters are used in infrared and ultraviolet photo-
graphy to absorb all wavelengths except those
intended to reach the film. Since these wavelengths
are outside the human visible spectrum, the resulting
images are often unusual. Often, infrared and ultra-
violet photography is used in the sciences to record
information not readily seen with the human eye.Filter Factors
By their nature, filters reduce the amount of light
striking the film. For the purposes of exposure
compensation, many manufacturers publish filter
factors for each filter, which determine how muchFILTERS