Cctv Lens - Accessory Or Necessity?...
The choice of lens depends mainly on the type of video surveillance camera. Among the most commonly used are lenses with fixed focus distance. Standard values vary from 2.8 to 16.0 mm. In so called zoom lenses, the focus distance is up to 70.0 mm. The less the focal distance is, the wider is the vision of the camera.
Fixed focus distance lenses like 2.8, 3.6, 4, 6 and 8 mm are suitable for small video systems. In rare and specific cases, such as surveillance of the area around the cash register in shops in order to achieve resolution to distinguish individual notes, 10, 12 or 16 mm are the best solution.
Zoom lenses are recommended for large sites where it is necessary to achieve high-resolution image of a great distance from the camera. For example, the entrances to stations or gates of factories and businesses where it is necessary to read the license plate number of the vehicle. Lens can be built into the camera or to be purchased separately. Choices are large and the optimum solution depends on the tasks to be assigned to the camera. Lenses are offered with fixed or variable focus, with options for automatic or manual focus.
Manually adjustable lenses are suitable for use inside the building and in combination with an electronic camera with regulating the amount of light falling on the sensor. This type of lenses is usually used in small spaces with constant level of illumination. It is believed that they are not suitable for installation in places which sunlight falls directly on the lens (near entrance doors or windows). Also, the absence of the diaphragm, which restricts the flow of light, makes a resulting image with low picture contrast even in good lighting. Therefore, this type lenses are not suitable for monitoring the area, which has objects at varying distance to the camera.