This short guide aims to help installers and engineers to calculate properly and understand contemporary functionality of the cameras that use the systems CCTV. The selection of the camera in CCTV systems is a very complicated process and depends on numerous parameters, such as if the camera is in color or monochrome, on the type of lens that will be used, or even from some special functions required by each application. Nowadays, most cameras offer DIP switches or keys that through them we can select the various functions we want each time.
The lens is one of the major components of a CCTV camera, because its bad selection significantly affects its attribution. An incorrect or poor selection is enough to destroy a CCTV system. The selection of the lens depends on many factors, like the physical location of the camera, the available light of the space, the type of image monitoring etc. The choice of the lens is usually made in accordance with the demands of the user, and whether it could be applied to a system. The most important criteria for selecting a lens are focal length and iris control.
Previously, larger cameras, such as 1 inch and 2/3 inch, used the adjustment type C lens for mounting the lens on the camera. With the advent of smaller CCD sensors such as ½ and 1/3 “CCTV industry adopted the adjustment type CS. The main difference in the dimensions of the two systems is the lens portion or flange, which sinks into the camera. Several cameras use a back focus mechanism of the lens, which allows the use of both types of lens adjustment. In contrast with type cameras CS, which can accept lens type C by using an appropriate converter, the cameras that are designed to operate with lens type C, do not work with lens type adjustment CS. Lenses with CS type adjustment is usually cheaper.
Sizes of lenses
The size of a lens is often noted as 1 “, W, 1/3”, which results from the ratio of the diameter of the lens to the size of the image that produced. In a camera the most common practice used is the size of the lens be similar to that of the CCD sensor. Furthermore, it is also possible to use lenses with bigger size in cameras with a smaller CCD sensor. The rule followed in selecting the size of the lens that will be used is that the image that will be produced from the lens should always match or be larger than the CCD sensor. The use of larger lenses in cameras with smaller CCD sensors is accompanied by some advantages. For example, in this way the user can increase the depth of field, while the image that will be produced by such a lens will has less distortion at the edges, compared with a smaller lens.
The focal length of a lens determines the field of view at a given distance. A wide angle lens, as the name implies, has a wide field at a given distance. This means that it can “see” a large area of the image in both the horizontal and vertical level. Because of this, the objects in the image will appear distant and will show few details. The opposite is that whichcorresponds to a telephoto lens. Generally, the focal length of a lens falls into two categories: fixed or variable.
Fixed focal length
The lenses that have a fixed length is usually more economical than those with variable. Of course, this has its effect because when the focal length is fixed, the viewing field will be fixed too. This means that you have to do accurate calculations to select a right lens for a given application. A change in the requirements of the application often results in changing the lens.
Variable focal length
Although these lenses are more expensive, the lenses with variable focal length are easier to use, adjustment and change. It is simple to get the correct viewing field of an image when it is possible to change the focal length and thus the viewing angle of the lens. The variable focal length should not be confused with the zoom lens , which have a larger adjustment range
The zoom lens is the next step after the lenses of variable focal length and offers the highest functionality. They can be continuously adjustable throughout the area in order to alter the focal length and viewing field, while they are usually remote controlled.
When a CCD sensor is exposed to a large amount of light the image will appear as rinsed. When a small amount of light falls on the CCD sensor, the result is that the image appears darker and loses the details, especially in shaded areas. For this reason, lenses use the iris, which is able to control the amount of light that falls on the sensor. The iris comprises a number of thin metal plates, which are disposed such as to produce a circular opening in its center. This opening, called iris diaphragm or iris may be smaller or larger, normally in fixed steps, called f-stops. Also in order to control the amount of light which enters the lens, iris has a secondary function in controlling the depth of field. The lenses may have a fixed, manually or automatically adjustable iris.
Fixed iris lenses
The fixed iris lenses cannot be adjusted for different lighting conditions. These lenses are more suitable for indoor lighting, where the light level remains constant. However, the electronic iris and the operation of automatic control can make this lens more flexible in use.
Manual iris Lenses
Manual iris lenses permit adjustment of the iris by the user, so that the camera can operate qualitatively in all light conditions. These lenses cannot react automatically to changes in the lighting of the image, for this reason they used as indoor cameras for the CCTV systems, where lighting environment remains stable. In this case the electronic iris and the operation of the automatic gain control, can enable in these lenses be used in a wide range of applications.
Auto iris Lensesς
For outdoor lighting, where the lighting of image is constantly changing, it is preferable to use a lens, which has a mechanism to adjust the iris automatically. The opening of iris (diaphragm) is controlled by the camera and is constantly changed to maintain the optimum light level for the CCD sensor. The lenses with auto iris usually fall into one or more types: automatic iris (AI), direct drive (DD) and galvanic driving.
Depth of field
The focus ring of a lens is usually adjusted so that the object of interest located within the image is properly focalized; to the extent that the objects that are behind and in front of this adjustment are focalized. The focus area also refers to as depth of field. As the objects fade out from the depth of field (further or closer to the lens), the proper focus disappears. The depth of field can be controlled by adjusting the iris of the camera. As the iris diaphragm reduces, the depth of field becomes larger, which means that most objects on either side of focus will be better focalized. One disadvantage arising from the increased depth of field is the reduction of the amount of light received by the camera, and therefore the image is darker. The depth of field depends on the focal length of the lens. The wide-angle lenses (e.g. those with smaller focal length) will have a larger depth of field than the telephoto lenses. The depth of field is inversely proportional to the focal length of the lens, as a result, as the focal length increases the depth of field decreases. The lenses with auto iris, because of the nature of their construction, cause changes in the depth of field. When the user adjusts the lens to the camera, he has to be sure that the iris of the lens is fully open. If the iris is closed, the increased depth of field can give a false impression to the user into believing that the lens is properly focalized, when in fact is not. This can be ascertained when the iris of the lens is open and the loss of focus will be observed.