CCTV: Back to basics: Part 1 | Archive Titles

Table 1: General examples of typical site coverage
General examples of typical site coverage
Small area	Room; entry hall; doorway
	Standard resolution camera. Wide angle lens, eg 3-4mm
Medium area	Small car park; large hall; shop sales floor; building front. High resolution camera to resolve small detail in the background. Medium angle lens 6-8mm
	Wide spaces
Large area	High resolution camera. Telephoto lens to zoom into detail or view the wider field
Specific detail	Identity faces/cars
	Ideally two cameras to resolve detail at a distance. Telephoto lens 12mm-plus. Also wide angle lens
Blind spots	Area under camera - the more narrow the lens angle, the larger the blind spot.Use a wide angle lens and site the camera as far as practical from the target, and horizontally
Angle of view	Lens	Deg	Coverage (typical)
	2.8mm	90	Full room
	4mm	62	2/3 room
	6mm	38	½ room
	8mm	30	building front

Table 2: What you should consider when choosing a camera
Black and white	Superior in low lighting environments. Provide good resolution. Can be matched with infrared (IR) lighting
Colour	Superior in identifying and tracking a target. Need good stable lighting conditions. Cannot work with infrared lighting
Day/night (twin colour/monochrome)	Colour cameras that switch to black and white when the light level falls below a prescribed level or when infrared lighting becomes present.Provide effective images in regard to the available light levels
Operating voltages
Mains voltage 230v AC	Used extensively in external applications but requires a fused spur at every camera position. The mains voltage can also be used as a power source to drive pan/tilt motors. Does not suffer from any measurable voltage drop
Low voltage 24v AC	Used in internal applications. Require a separate power supply/transformer but do not suffer any significant voltage drop
Low voltage 12v DC	Similar to 24v AC systems but more prone to voltage drop on extended cable runs over 100m
	Larger systems need a number of power supplies installed
Line fed	Cameras are inherently small and compact as the power for the unit is supplied on the coaxial signal cabling. Installation is easy
	These are dedicated units with little flexibility for system expansion and are often supplied as DIY products. More specialist cameras can be found in covert operations or in discrete housings

Table 3: What you should consider when choosing a lens
	Note: The focal length of the lens, which is measured in millimetres, determines the actual field of view at a given distance. In practice this can be calculated, but it is normal to use manufacturers' rotary 'Lens Selector' calculator devices that determine the required lens focal length from the object distance to the camera, and the angle of view or scene size. Short focal lengths give a wide angle of view. As the focal length increases, the view of the camera becomes progressively narrower
Fixed lens	Have a fixed focal length. Used where the field of view of a scene has been clearly established and no further adjustment is required
Varifocal lens	Have variable focal lengths and are adjustable manually. They allow the field of view of a scene to be adjusted over a small specified range. Used where a standard lens is not available with the required focal length or where a margin of error is required in the installation set-up
Zoom (telephoto) lens	Have a long focal length to produce high magnification but a narrow angle of view. Allows an operator to automatically change the field of view from wide angle to telephoto view. The zoom ratio refers to the range of alterations allowable within the range of focal lengths
Manual iris	Only to be used where the lighting levels remain constant. Prominent in internal applications
Auto iris	The iris automatically adjusts to compensate for changes in the light levels. The control of the iris is managed according to the image analysis of the video signal. Used where fluctuations of light levels occur, such as in external installations
	These may be direct drive, which uses the electronics within the camera or video drive, and have an amplifier built into the lens, drawing the low power needed to control the minuscule motors from the camera
Aspherical lens	Used to achieve better sensitivity levels with low light cameras
	Note: The correct electrical connections are needed between the various lens drive circuit and lens. For small installations, fixed focal length and varifocal lenses are popular with direct drive or motorised iris control.Check for manufacturer compatibility.