Many traditional surveillance cameras do not come with a lens. As such, it is important to understand the different lens types and their related terminology. The three main types of lenses are based on focal length and are normal, wide-angle, and telephoto. Focal length is defined as the distance from the center of the lens to the surface of the CCD sensor of the CCTV camera. It is the point at which light rays focus when the lens is fixed on an object at infinity.
A 5-50mm Manual Iris Surveillance Camera Lens by Fujinon
The focal length of a camera is given in millimeters and it can be fixed or variable. Fixed focal lengths do not change. Lenses with fixed focal lengths are the most economical and are ideal for situations where the illumination never changes, such as an office. A lens with a manually adjustable focal length is called a varifocal lens. These lenses are good if the light changes often. Variable lenses that are electrically powered are called zoom lenses. Most CCTV lenses vary from 3.6mm to 50mm for fixed focal length lenses and can exceed 70mm for zoom lenses. Varifocal lenses actually have a ratio or a focal length range from the telescopic end to the wide-angle end. The greater the ratio, the greater the range of view. Zoom lenses also have a ratio but changing the focus is achieved with the use of a motor. Cameras with zoom lenses sometimes also come bundled with pan and tilt features, all of which are motorized.
Field of View
When discussing lens applications, it is important to know the field of view. The field of view is the angle of which objects are captured by a camera’s sensor. It depends on two things: the focal length of the lens and the size of a camera’s sensor. There are basic rules that apply when understanding a camera’s field of view. One is the shorter the focal length, the wider the field of view. Likewise, the longer the focal length, the narrower the field of view. Also, the smaller the CCD sensor, the narrower the field of view. 30 degrees is considered a standard field of view because it corresponds to what the human eye sees as normal. The average field of view for a normal lens is 50 degrees. The objects appearing in the field of view appear normal in size and shape. The average field of view for a wide-angle lens is 90 degrees. Short focal length lenses of up to 4mm have wide fields of view. Large scenes, when viewed with a wide angle lens, have less detail. These lenses are generally used for close up observation. Telephoto lenses have a smaller field of view than normal lenses and show distant objects with greater detail.