CCTV can use fixed focal length or varifocal lenses. Which of the following correctly describes the lenses used in CCTV?

A.
A fixed focal length lens allows you to move between various fields of view with a single lens.

B.
To cover a large area and not focus on specific items, use a large lens opening.

C.
An auto-iris lens should be used in an area with fixed lighting.

D.
A shallow depth of focus allows you to focus on smaller details.

Explanation:
D: A shallow depth of focus allows you to focus on smaller details as opposed to a larger field. To understand depth of field, think about pictures you might take
while on vacation with your family. For example, say you are on the beach on the Hawaiian island of Oahu with your family and you want to take their picture at the
shoreline. Because the main object of the picture is your family, your camera will zoom in. This shallow depth of focus provides a softer backdrop, leading the
viewer’s eye to the foreground of the photograph. Now you want a scenic picture of Diamond Head. Your camera uses a greater depth of focus, lessening the
distinction between objects in the foreground and background.
A is incorrect because a fixed focal length lens must be changed to get a different field of view. Fixed focal length lenses are available in wide, medium, and
narrow fields of view. A lens that provides a “normal” focal length creates a picture that approximates the field of view of the human eye. A wide-angle lens has a
short focal length, and a telephoto lens has a long focal length. When a company selects a fixed focal length lens for a particular view of an environment, it should
understand that if the field of view needs to be changed (from wide to narrow, for example), the lens must be changed.
B is incorrect because it is best to use a wide-angle lens and a small lens opening to get the correct depth of field for a large area. It is necessary to understand
the depth of field when choosing the correct lenses and configurations for your company’s CCTV. The depth of field refers to the portion of the environment that is
in focus when shown on the monitor. The depth of field varies depending upon the size of the lens opening, the distance of the object being focused on, and the
focal length of the lens. The depth of field increases as the size of the lens opening decreases, the subject distance increases, or the focal length of the lens
decreases.
C is incorrect because an auto-iris lens should be used in environments where the light changes, as in an outdoor setting. As the environment brightens, this is
sensed by the iris, which automatically adjusts itself. A manual iris lens should be used in an area with fixed lighting. Manual iris lenses have a ring around the
CCTV lens that can be manually turned and controlled. A lens with a manual iris would be used in areas that have fixed lighting, since the iris cannot self-adjust to
changes of light.