When we view common objects under
different light sources, our vision makes subtle adjustments so that
white appears white, black appears black, and other colors their
expected hues, regardless of the light source. However, color
photographic film has no such capability.
Most of us realize that tungsten lighting
in the form of studio photo floods or ordinary house bulbs emit light
rich in orange and red. The illumination produced by each of these
sources is known as warm light. Contrasted to this, sunlight and
electronic flash tubes give off light generically termed "daylight."
This type of light is full-spectrum, but at certain times appears
blue-rich and therefore cool. When taking pictures under specific light
sources, color film must be matched to the light source to produce
accurate hues. We can accomplish this in by either filtering the light
to match the spectral response of the film or to match the spectral
response of the color film to the light source.
Our eye adjusts for different light sources automatically. Film does
not. For example, shooting with daylight-balanced film under tungsten
lighting results in pictures displaying an overall orange cast. An
orange color rendition might be perfectly all right and even desirable
on some occasions, but usually it isn't. As stated earlier, we can use
a filter to address the imbalance or switch to a film matched to the
Color film marked "daylight" is intended for use under natural daylight
or electronic flash. Film labeled "tungsten" is normally employed for
indoor situations where the principal light sources are either floods
or other warm tungsten-light sources.
Indoor red-rich light has a low color temperature compared to blue-rich
daylight. This seeming contradiction occurs because tungsten films
designed for use with warm, red-rich lighting have smaller Kelvin
numbers than cooler daylight films balanced for blue light. This
situation is a constant source of confusion to many but we should
understand the terms warm and cool are purely psychological
descriptors, while the numerical designations of color temperature as
expressed in degrees Kelvin are more correct scientifically.
Color film is most often used under compatible lighting. In order to
provide photographers with color films that deliver realistic color
under light other than daylight, major film suppliers make a number of
color films, almost all of which are transparency materials whose color
response is balanced for tungsten light. If you need to match color
exactly, you will need a color temperature meter, an instrument that
measures prevailing light and reads out its color temperature. After
taking a color temperature reading, placing one or more special
color-correction filters over either the light source or the taking
lens should balance film and light source precisely.
At the cool end of the visible spectrum, color accuracy is a hit easier
to achieve because large changes in color temperature do not result in
as much of a color shift as at the lower end of the color temperature
scale. Aware that color temperature meters and color compensating
filters are both expensive and cumbersome, manufacturers make several
slide films formulated to give normal color balance under several
sources of tungsten light. There are a half-dozen such films that range
from ISO 50 to 640. All but two are balanced for studio floodlights
burning at 3200K. They are designated Tungsten Type A. One film,
Kodachrome 40, balanced for infrequently used 3400K photolamps, is
designated Tungsten Type B. Fuji has chosen to balance their tungsten
films for a 3100K light source. They believe 3100K is closer to the
actual color temperature of photo floods.
Tungsten film is useful under ordinary house lights although 60-150
watt bulbs burn at between 2300-2800K which is warmer (more orange)
than photofloods. The resulting pictures will be warm but not always
objectionably so. Tungsten film can be used in daylight with an 85B
filter for a more normal color rendition.