Light Modifiers

angle of incidence angle of reflectance
DIAGRAM 1-7. Light travels in a straight line. When it strikes a
surface, the angle of incidence equals the angle of reflectance.
This concept is key to lighting your images successfully.
Direction. The placement of the main light source relative
to the subject is another controlling yet subjective factor in
the quality of the light in your final portrait. Light travels in
a straight line, and when it strikes a surface, its angle of incidence
is equal to its angle of reflectance. For example, with
front lighting, the light is directly at or behind the camera,
producing a zero-degree angle of incidence and thus a zerodegree
angle of reflectance. The result is flat lighting—an
image that has no distinct transitions from shadow to highlights.
For example, if the light source is moved 45 degrees
to the left or right, off the lens axis, it will reflect back at the
same 45-degree angle. This concept is important when positioning
your light and modifiers to show shape and create
depth on your subject.
THE INVERSE SQUARE LAW
Understanding the Inverse Square Law is very important to
becoming skilled in the art of photographic lighting. This
law describes how a change in the distance of the light source
to the subject will change the intensity of the light on the
subject. The Inverse Square Law states that when the distance
of the light to the subject doubles (2x the distance),
the light on the subject will be reduced to 1/ 4 of its original
intensity—because the inverse square of 2 is 1/ 4 (1/22 ). This
means that when you change the distance of the light to the
subject, the exposure (or the output of the light) must be
adjusted to compensate.
26 THE DIGITAL PHOTOGRAPHER’S GUIDE TO LIGHT MODIFIERS
f/5.6 f/5.6
Distance of
subject to light
doubled, light
intensity
reduced to
1 /4 of
original.
light source remains
at constant setting at
camera axis
Distance of
subject to light
halved, light
intensity
increased to
4 times
original.
f/11 f/11
DIAGRAM 1-8. The Inverse Square Law. If you move your subject
from 3 feet to 6 feet from the main light, you’ll need four times
the amount of light to arrive at the same exposure—two full stops
more light. The beam of light widens as it travels farther from the
source; because it illuminates a broader area, the light becomes
less intense.