The
following terms and definitions apply to closed circuit TV (CCTV)
for video monitoring and surveillance.
Depth of
Field - Depth of field is the area in focus in front of and
behind an object. When an object is in focus, a certain distance in
front of and behind the object will also be in focus, although exact
focus occurs only at the precise focusing distance. It can be improved
by increased lighting or the use of regular lenses rather than telephoto
lenses.
F-stop - The ƒ number is the ratio
between the size of the aperture and the focal length of the lens. Each
f-stop means that the same measurement of light is reaching the camera.
Each change in f-stop indicates either a doubling or halving of the
amount of light. The ability of a lens to gather light depends on the
relationship between the lens opening (aperture) and the focal length.
Usually the F-stop can be found printed on the side or front of the
lens. The lower the F-stop number, the larger the maximum lens aperture
and the greater the lens ability to pass light through the camera.
Focal Length - Focal length, measured
in millimetres, it is defined as the distance from the optical center
of the lens to a focal point near the back of the lens. It indicates
the field of view produced by the lens. Fixed focal length lenses are
available in various wide, medium, and narrow fields of view. A lens
with a normal focal length 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. A fixed
focal lens has a constant focal length, while a varifocal or zoom lens
can change its focal length.
FOV
- acronym for Field of View. The FOV is the actual picture size (height
and width) produced by a specific lens. Tables are available to calculate
the proper image size, lens and distance combination needed to produce
a desired field of view. In general short focal length lenses have wide
fields of view. This is good for close-ups or for seeing a large area.
Objects appear smaller, rapidly, as distance from the camera increases.
As lens focal length increases, the field of view narrows and more distant
objects are easier to define. Different lens (wide angle, telephoto,
etc.) are used to increase or decrease the field of view.
Lens Mount
- Camera lenses generally come with either a C-mount or CS-mount and
must be matched appropriately to the camera's mounting requirements.
A C-mount is the simplest type of mount on the market and just screws
into the camera. Modern cameras and lenses are generally CS mount rather
than C mount. A CS-mount lens cannot be used on a C-mount camera.
PTZ
- acronym for Pan/Tilt/Zoom. PTZ cameras are motor driven and can pan
left and right, tilt up and down and zoom in and out for close-up or
wide-angle viewing.
Video Switcher
- A video switcher enables separate cameras to be switched to separate
monitors. Switchers are available as a manual switcher that allows a
users to select a camera by pressing a button, or a automatic switcher
that that will switch one camera to another automatically. A matrix
switcher allows an operator to switch any camera to any monitor.
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