Surveillance System Camera field of view

Surveillance System Camera lenses and field of view

Lenses . A camera lens focuses light reflected from objects within a scene onto the imaging device of the camera. The imaging device converts light to an electrical signal. Lens focal length and aperture are two important parameters to consider.

Lens focal length describes the relative magnification of the lens. The camera field of view (defined below) will be dependent on the lens focal length, along with the camera imager format size. Similar to the camera imager format, there is a format size for lenses. For most cases, the lens format size should be matched to the camera imager format size. Mismatched format sizes can result in the focused image being too large or too small for the camera imaging device. Different camera and lens formats can be used satisfactorily in a few instances.

Except for the most uncommon sizes, there usually is not a large price difference between various lens sizes. The most common sizes are 4.8mm, 5.6mm, 8mm, 12mm, 16mm, 25mm, and 35mm. A 35mm lens has the longest range with the narrowest field of view. The 4.8mm lens can see much shorter distances, but it will have a much wider field of view. Most lens sizes can be used in exterior applications, depending on the view desired. Shorter focal length lenses, such as 4.8mm or 5.6mm, are typical for interior applications, due to the shorter distances involved.

The important thing to consider is that the camera field of view depends on the focal length and format size. Camera field of view is expressed in horizontal and vertical angular fields of view. Most camera manufacturers or manufacturers' representatives who sell lenses with their cameras can provide charts that list the angular fields of view for common lens sizes. The image to the right shows the difference between two different lens focal lengths. The left portion of the image uses a 6mm lens while the right portion of the image uses a 60mm lens to provide for a manual zoom.

The lens aperture, or speed of a lens, is a relative measure of the ability of the lens to gather light. Aperture is expressed as the F-number. The F-number is the ratio of lens focal length to its clear aperture. Clear aperture is the diameter of the inside of the lens where light passes through when the lens iris is fully open. A lens that is designated as an F/2 will have a clear aperture size that is one-half its focal length, meaning that a 16mm focal length lens will have a clear aperture of 8mm. The lower the F-number of a lens, the more light the lens can gather. This becomes important when operating a camera at low light levels, such as at night with artificial lighting. Most security camera lenses today have F-numbers of 1.8 to 1.4. These are usually adequate for night applications given that the minimum light levels for CCTV are provided.

Not all lenses are the same, however. Two different lenses with the same F-number can have different light-gathering capabilities. This is particularly true when it comes to fixed focal length lenses versus variable focal length (zoom) lenses. Zoom lenses have more glass elements than fixed focal length lenses. Because of the additional glass elements, an F/1.8 zoom lens will not be able to pass as much light as an F/1.8 fixed lens with fewer glass elements. An amount of light transmission is lost in each glass element. This is important to consider during night operation under artificial lighting. A zoom lens will require higher lighting levels than a fixed focal length lens if an equivalent picture quality is desired.

Most lenses for security cameras will have an adjustable iris to control the amount of light that is received at the camera imager. The iris is either manually adjustable or electronically controlled. The electronic iris (or auto-iris) monitors the camera video signal output and will open the iris for decreasing light levels and close it for increasing light levels. This keeps the video level (brightness and contrast) fairly constant under varying lighting conditions. In the case of a manual iris lens, the user or installer adjusts the iris opening for the proper video signal level for the expected operational lighting level. If light levels change, an adjustment to the iris will be required in order to maintain a proper video signal level. Manual iris lenses are used mostly in interior applications where no outside light comes in and the light levels remain constant. For all exterior and many interior applications, an auto-iris lens will be necessary.

A relatively new feature in many cameras is the electronic shutter. The electronic shutter is part of the imaging device and can perform close to the same function as an electronic iris. It controls the amount of light that the light-sensitive elements within the camera imager receives. Electronic shutters have limitations, however. They may not have as much range as auto-iris lenses. This is an important consideration for exterior applications. If light control is totally dependent on a shutter (a manual iris lens is used instead of an auto-iris) in an exterior application, the shutter may not be able to reduce light enough on bright, sunny days, resulting in portions of the picture washing out. If the manual iris lens is partially closed to compensate for bright sunshine, low-light conditions may produce a dark, noisy picture. Many shuttered cameras intended for exterior use will also come with an auto-iris lens.

Field of view . Field of view (FOV) relates to the size of the area that a camera will see at a specific distance from the camera. The field of view is dependent on lens focal length and camera format size.

Before the FOV for a camera is selected, the minimum desired resolution for an intruder or object to be viewed must be determined (i.e., whether it is desired to identify a person or to just determine if a person is within the scene). This will limit the maximum FOV width and is referred to as the resolution-limited FOV (image to the right). The resolution-limited FOV width can be determined by using camera resolution in horizontal lines per foot and the number of lines of resolution per foot required to identify an intruder. The following formula is used to calculate the resolution-limited FOV width:

A resolution of 16 lines per foot is considered acceptable for identifying most people. If a camera with 350 horizontal lines of resolution is utilized, the resolution-limited FOV width for a resolution of 16 lines per foot can be calculated.

A viewfinder can also be used to determine the field of view of a lens. This is a specially designed lens through which one can view the scene of interest. The scene is masked through the lens in such a way as to represent the picture that will be seen on the monitor. The scene desired can be dialed up on the viewfinder and the focal length of the lens required for the particular imager format size of the camera read from the side of the viewfinder. A viewfinder only determines a lens focal length value; other parameters must still be calculated.

Some lens manufacturers have developed tables for determining the field of view. The format size and focal length of the camera is cross-referenced to the column of the desired distance, and the width/height of the field of view is read from that column.

In summary, whether a camera scene is useful depends on whether objects can be distinguished in the scene. Camera resolution, camera format size, lens focal length, as well as lighting, shadowing, camera aiming, and camera sensitivity all play a role in being able to distinguish objects. Resolution and performance of other components such as TV monitors, recorders, and signal transmission equipment must be considered also. Cameras are specified with the number of horizontal lines of resolution and active picture elements. Most security cameras available today range from 300 to 700 horizontal lines of resolution. Black-and-white security cameras commonly have a horizontal resolution of 500 to 600 lines, while color cameras for security applications have 300 to 400 lines. In many exterior applications and some interior applications, a greater number of low-resolution (200-300 lines) cameras may be necessary in order to distinguish objects than would be necessary using higher resolution (500-600 lines) cameras