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The Lens (Optics)
Lenses (Optics) play an important role in the design of a CCTV system. Their
primary function is to
collect reflected light from a scene and focus a clear, sharp image on the
camera's imager. Typically, the more light that passes through a lens, the
better the quality of the picture.
Selection of a lens is especially critical because it directly affects the size,
shape, and sharpness of the image to be displayed on the imager. Factors such as
distance from the scene, focal length, desired field of view, lighting and
format affect the size and clarity of the image on the camera's imager.
Field of View
The field of view (FOV) is the actual picture size (height and width) produced
by a specific lens. If the field of view is not suitable, you may consider using
a different lens (wide angle, telephoto, etc.) to increase or decrease the field
of view. Tables are available to calculate the proper imager size, lens and
distance combination needed to produce a desired field of view. See page 92 of
this catalog.
Camera lenses can be divided into two basic types: fixed focal and varifocal (or
zoom). A fixed focal lens has a constant focal length, while a varifocal lens
can change its focal length. Focal length is simply the distance from the
optical center of the lens to a focal point near the back of the lens. This
distance is written on the lens (in millimeters) and indicates the field of view
produced by the lens (See figure 7)
Figure 7: Focal length.
Fixed focal length lenses are available in various
wide, medium, and narrow fields of view. A lens with a "normal" focal length
(Ex: 8.0mm on a 1/3" camera) produces a picture that approximates the field of
view produced by the human eye. A wide-angle lens has a short focal length,
while a telephoto lens has a long focal length (See figure 8). When you select a
fixed lens for a particular field of view, bear in mind that if you want to
change the field of view, you must change the lens.
Figure 8: Wide angle vs. telephoto.
When both wide scenes and close-up views are needed, a
varifocal or zoom lens is best. A zoom lens is an assembly of lens elements that
move to change the focal length from a wide angle to telephoto while maintaining
focus on the camera's imager. This permits you to change the field of view
between narrow, medium, and wide angles, all on one lens.
F-Stop
The ability of a lens to gather light depends on the relationship between the
lens opening (aperture) and the focal length. This relationship is symbolized by
the letter f, which is commonly referred to as the "F-stop," and can be found
printed on the side or front of the lens (see figure 9). The lower the F-stop
number, the larger the maximum lens aperture and the greater the lens' ability
to pass light through to the camera's imager.
For example, a lens with an F-stop of f/1.2 can gather
a great deal more light than a lens with an F-stop of f/4.0. A lens with a low
F-stop number is called a "fast" lens
Figure 9: The F-stop indicates the lens' light gathering ability.
Depth of Field
Another consideration when determining the proper lens is depth of field. Depth
of field is the area in focus before and behind a subject (see figure 10). This
means that when you focus precisely on a subject, a certain distance in front of
and behind the subject also will be in focus, although not as sharp. Depth of
field increases or decreases based on the 1.) Length of the lens, 2.) The lens
aperture and 3.) Distance from the camera to the subject.
Figure 10: Depth of field.
Each of the three depth of
field factors will yield the following:
| 1) Lens length |
Short lens (i.e. wide angle)
|
= longer depth of field |
| Long lens (i.e. telephoto) |
= shorter depth of field |
| 2) Aperture |
Wide aperture (low F-stop)
|
= shorter depth of field |
| Narrow aperture (high F-stop) |
= longer depth of field |
| 3) Distance to subject |
Short distance
|
= shorter depth of field |
| Long distance |
= longer depth of field |
Purchasing and planning decisions should take these
factors into account since depth of field can affect image quality (and may
jeopardize the ability to identify and prosecute subjects). If depth of field is
important, you may want to explore options such as increasing artificial
lighting or installing cameras with normal lenses rather than telephoto lenses,
etc.
Lens Mounts
Camera lenses generally come with either a C-mount or CS-mount and must be
matched appropriately to the camera's mounting requirements. The difference
between the two mounts is the distance of the lens optics from the camera
imager. The C-mount lens is 17.5mm from the imager; the CS-mount lens is 12.5mm
from the imager.
Follow these guidelines when purchasing equipment:
| • |
A C-mount lens can be used on a CS-mount camera only if a 5mm spacer
ring is added |
| • |
A CS-mount lens cannot be used on a C-mount camera |
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