Figure 1-12.–Effects of sunlight passing through the atmosphere.
forms of light, direct or indirect, that originate from the
sun.
Of importance to the photographer is the effect of
the atmosphere on sunlight and the amount of
atmosphere through which sunlight passes (fig. 1-12).
The shorter wavelengths of light (violet and blue)
are scattered by the atmosphere much more than the
longer wavelengths. The color composition of sunlight
becomes increasingly deficient in blue the further the
light has to travel through the atmosphere (early
morning and late afternoon). As the sunlight becomes
more deficient in blue, it appears more yellow. The
amount of scattering also depends on the condition of
the atmosphere. When the atmosphere is clean (has little
moisture or fine dust in it), there is less scattering than
when the atmosphere is hazy or dirty (having a good deal
of moisture or tine dust and smoke). The variation in
color of sunlight can be expressed as color temperature.
Sunlight coming from overhead on a clear day has a
color temperature of about 5400 K. Just after sunrise and
just before sunset, the color temperature ranges between
2000 K and 4000 K. Not only is the color of sunlight
different early in the morning and late in the afternoon,
but the intensity is also less. These arc important
considerations when taking pictures at these times of
day.
Light scattered by the atmosphere, or skylight as it
is called, can be regarded as a second source of light.
Skylight is different than sunlight because it is caused
chiefly from the scattering of the shorter wavelengths.
It therefore appears more blue than sunlight. Skylight
on a clear day may be as high as 60000 K.
ARTIFICIAL LIGHT
The types of artificial lighting you use in
photography give you complete control over the
direction, quality, and strength of the light. You can
move these light sources around, diffuse them, or reflect
them. You can alter their intensity to suit the situation.
There are two types of artificial light sources:
spotlights and floodlights. Spotlights provide a
concentrated beam of light. Floodlights give diffused,
softer, more even, spread out light. You can add to these
two basic types of artificial light sources. By using
lighting accessories, such as reflectors, barn doors,
diffusers, and snoots, you can control the light to provide
a variety of lighting effects.
Unless special effects are wanted, artificial light
sources that are different in color temperature or quality
should not be mixed (used together). When you are
viewing a scene, your eyes adapt so color differences
between two or several light sources are minimized.
Color film, however, cannot adapt and shows the color
difference in parts of the scene illuminated by different
light sources.
Tungsten-Filament Lamps
Tungsten light color films are made to be used with
tungsten-filament light sources and are color balanced
for 3200 K or 3400 K. Tungsten lamps, operated at their
rated voltage, produce light of 3200 K and 3400 K. The
color temperature of tungsten lamps changes with
voltage fluctuations, decreasing with lower voltage and
increasing with higher voltage. For example, the color
temperature of a tungsten lamp rated for operation at
115 volts increases about 10 K for each 1 volt increase.
Usually, a variation of less than 100 K has no adverse
effect on the rendering of scene colors. However, a shift
as low as 50 K can be noticeable on subjects with
important neutral areas, such as white and light shades.
When you are using tungsten lamps, the color
temperature can shift, depending on the amount of
power being drawn on the same circuit. When possible,
you should avoid having other equipment on the same
circuit. For these lamps to produce light of the correct
color, they must be operated at exactly their rated
voltage. When it is not possible to operate the lamps at
their proper voltage appropriate filters can be used to
correct the color of the light reaching the film.
1-8
