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Television Production & Broadcast Journalism
Copyright Goodheart-Willcox Co., Inc.
The iris is comprised of blades that physically expand and contract.
The movement of these blades adjusts the size of the opening that allows
light to pass through the lens, Figure 3-17. A camera’s iris operates much
like the iris of the human eye. As the size of the iris increases, light is
blocked from passing through to the CCD. When the iris contracts, more
light is allowed to pass through.
Many consumer and professional cameras have an auto-iris circuit,
as well as a manual iris control. The auto-iris circuit examines the light
iris: A component of a lens
that is comprised of blades
that physically expand and
contract, adjusting the
aperture size.
auto-iris circuit: A feature
on many consumer and
professional cameras that
automatically examines the
light levels coming into the
camera and adjusts the
iris according to generic
standards of a “good” picture.
Goodheart-Willcox Publisher
Figure 3-17. The size of the iris determines the size of the aperture. A large iris creates a small aperture;
reducing the size of the iris produces a larger aperture.
Small
iris
Large
aperture
Large
iris
Small
aperture
f/2.8 f/1.8 f/4
f/11 f/8 f/22
room. When you exit the theater and step into the bright daylight, you squint
and the iris expands. This makes the pupil smaller and reduces the amount
of light hitting the rods and cones. If the iris does not expand enough to
sufficiently reduce the amount of light hitting the retina, you continue to
squint until you get a headache or find sunglasses to further reduce the light
hitting the retina. The television camera lens is expected to operate the same
way as the human eye when reproducing colors and tones and reacting
to lighting changes in the environment. Even though it valiantly tries, a
television camera lens does not succeed in functioning as well as the human
eye. The camera lens needs a human to help it operate.