Surveillance-camera-lenses,how to take an informed choiceHome > Camera Equipment > Camera Lenses Surveillance-camera-lenses determine the quantity of light received by the camera. The relationship between lens opening (aperture) and focal length establishes the amount of light collected through them. Home Surveillance and Security Click on Course Invitation. (Sponsored Links) Why are Surveillance-camera-lenses so important? As lenses are interchangeable, changing them will change the Surveillance camera lenses are usually separate from the camera and, for more expensive products, can be purchased separately, according to the application. Off the shelf cameras come with a certain lens: customers should be advised that they are probably offered the most popular choice of lens for a certain camera. However, if a selection is advisable, one should understand the principles in order to choose one of the most suitable Surveillance-camera-lenses for the application. It is understood that most of the following, although intended here specifically for Surveillance-camera-lenses of Surveillance Camera systems, applies also to Photography in general and to photographic cameras of traditional type that use film as recording medium. The three basic types...
Focal length is actually the distance from the center of the Surveillance-camera-lenses to the surface of the solid-state target imager of a CCTV camera, the point at which light rays converge or focus when the lens is focused on an object at infinity. Focal length can be either fixed or variable. The fixed focal length Surveillance-camera-lenses are relatively economical. Manually variable focal length lenses are called Vari-Focal Camera Lenses. Electrically powered variable focal length lenses, suitable for control from a distance, are called Zoom Surveillance-camera-lenses. These are generally too expensive for use in most Home Surveillance camera systems. The focal length of a lens is usually given in millimeters (mm). Focal lengths of most CCTV lenses vary from 3.6 mm to 16 mm for fixed focal length lenses, to over 70 mm for zoom lenses. Varifocal Lenses The zoom ratio is ratio between the telescopic end focal length to that of the wide-angle end. The greater the ratio, the larger the range available for sizing the image. This more expensive selection eliminates the need to pick up specific Surveillance-camera-lenses of given fixed focal length, with more versatility of use (at a price) than simpler ones. Motorized Zoom Lens This feature makes the selection even more expensive. It makes sense only if the surveillance is performed in real time, with a person dedicated to obtain always the most significant pictures. Cameras with motorized zoom Surveillance-camera-lenses are normally used in combination with motorized pan/tilt heads, with all operations under remote control. Field of view As it depends on the sensor size, it is not a fixed characteristic of a lens and it can only be stated if the size of the sensor it will be used with is known. With a normal lens, the viewing field is approximately 50 degrees. The objects photographed appear normal in size and shape, relative to the picture's background. In general, short focal length lenses up to about 4 mm have wide fields of view.In a wide angle field of view a larger scene is observed with less details. In a wide angle lens, the field of view is much wider: about 90 degrees. These Surveillance-camera-lenses are used where the distance between camera and subject is limited, or to photograph large objects from close up. Telephoto, or long focal length Surveillance-camera-lenses, have a smaller field of view than a normal lens, and show more distant objects in more detail. Fixed focal length lenses come in standard, wide-angle and telephoto types. Iris, diaphragm, aperture The amount of light allowed to pass through Surveillance-camera-lenses depends on the size of the opening which adjusts the quantity of light rays passing through. The aperture is the camera adjustment that controls how much light enters the camera. Mechanically reducing the aperture improves optical performance, particularly towards the edge of the picture, and increases the Depth of Field, which is the zone of good focus. Depending on the sensitivity of the imaging medium (film, sensor etc.) reducing the aperture may require lengthening of exposure time, in order to obtain a similarly bright picture. To measure the size of an aperture opening, the lens industry adopted the F-stop measurement that also defines the optical speed, or light-gathering power of the lens. Generally, the higher the f/stop number, the slower the optical speed, the less light passing through the camera lens and the greater the Depth of Field. The f-number (focal ratio), as used in photography, is used to express the diameter of the diaphragm aperture in terms of the focal length of the lens. For example, f/8 represents a diaphragm aperture diameter that is one-eighth of the focal length. Conversely the focal length is 8 times the aperture. Conventionally the f-stops are arranged in geometric progression, so that any number represents an aperture whose area is one half of the preceding one. Consequently the passing light intensity is half that of the preceding f-stop. In a fixed-iris lens, the aperture is set and cannot be changed. This may be a limitation but is also a great advantage, because the Home Surveillance Camera-lens is set once and for all. CCTV surveillance cameras that operate in constant light conditions (inside buildings with fixed artificial light) will perform satisfactorily with a fixed iris lens and with fixed focal length. These are the least expensive and the most widely used. Manually adjustable iris-type diaphragm, although practical for a photographer, is not suitable for a security camera that must work unattended. Therefore useful choices for a Home Surveillance Camera pointed to outdoor scenes in full daylight, where the light intensity can vary continuously, are Surveillance-camera-lenses including auto-iris feature that automatically adjust the amount of light that passes through them. This is done with a motorized aperture under electronic control that adjusts itself according to the amount of light that's available at the time. Even an automatic iris lens can be only be set for the overall light level, with some compensations for different contrasts within a scene. Exposure Control The exposure is thus 1/25 of a second for mains frequency of 50 Hz (Hertz or cycles per second) and 1/30 of a second for mains frequency of 60 Hz. The exposure time is therefore fixed and the only control of the amount of light passing to the sensor is by adjusting the size of the iris. However a few automatic adjustment features are available (at a price) that tend to improve performance under changing illumination conditions. These are briefly presented here for information, although not necessarily recommended, in case of a normal home surveillance application. Backlight Compensation (BLC) Wide Dynamic Range permits to double the standard number of exposures for each frame described above and to control the length of time that the automatic electric shutter (AES) is open. The resulting two images per frame are then synthesized so that both dark and bright subjects are more clearly reproduced. Automatic Gain Control adjusts the video signal electronically to maintain a more constant level of screen brightness. Other more specialized functions are sometimes offered, suitable for more consistent budgets available to large organizations. With future development and price reductions, many of the improvements are likely to find their way as standard features included in low end products. Lens Mounts Surveillance-camera-lenses generally come with either a C-mount or CS-mount. They have the same thread pitch and diameter. The difference between the two mounts is the distance of the lens optics from the camera imager. The lens distance provided by a C-mount is about 17.5 mm from the imager, the shorter CS-mount lens is only 12.5 mm from the imager. As cameras are made increasingly smaller, CS mounts with their shorter lens distance are becoming more popular. A lens built for a C-mount can be used on a CS-mount camera only if a 5 mm spacer ring is added. A CS-mount lens cannot be used on a C-mount camera. The bayonet mount is often used for 3CCD type cameras (see Camera Sensors), and has no interchangeability with either C or CS mounts. In summary for an economic Home Surveillance camera one should probably select (or verify that the lens included meets the requirements) a fixed focal length lens of the required type (normal, wide angle or telephoto) and with fixed iris for constant light, or auto iris for widely varying lighting conditions. If the purposes of the surveillance warrant a more definite identification capability and if cost is not an issue, then more complex solutions may be looked for as we are going to explain in more detail in our pages.
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