نوع مطلب :مقالات آموزشی ،
(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)
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.
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.
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
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.
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)
depth of field
Long lens (i.e. telephoto)
= shorter depth of field
Wide aperture (low F-stop)
depth of field
Narrow aperture (high F-stop)
= longer depth of field
3) Distance to subject
depth of field
= 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.
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.
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