288 Electronic Definition or Meaning for cctv lenses


Definition for cctv lenses


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CCTV LENSES
What is cctv lenses in electronics?
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cctv lenses

There`s an old saying which runs: lf you buy cheaply you`ll pay twice!" and that sums up the whole business of buying lenses for CCTV cameras perfectly. My experience shows that if you make economics on the purchase price of CCTV lenses you`ll find that made a big mistake later. if we accept that the best way to devise a system is to specify the lens and work backwards then we must appreciate that the lens is the single most important element of any CCTV system. if the camera does not receive a usable image the whole system`s credibility is brought into question.
Of course visual monitoring installations are no longer the sole property of security and law enforcement industries. Large centre managers retail "centre owners and urban planners have all discovered how the benefits of visual surveillance can make their jobs easier and more effective. More and more end users are now better informed about how they want an installation to perform but they have to be told that CCTV cameras do not come in a box with a lens attached. The lens is ihe first part of the system to encounter and transmit images and if it is incorrectly specified then the most liightech cameras and top quality monitors could all fail to perform adequately. And so it is vital that the installation engineer is aware of basic lens functions and the various lens types available.

Focal Length and Field of View
The focal length of the lens required is determined by exactly what the end user wants to see and from what distance he wants to see it. The field of view varies along with the focal length: a lens with a short focal length gives a wider angle view a lens with a longer focal length has a narrower
field but can see things farther away. irrespective of camera format the focal lengili of the lens remairis an absolute constant. However the field of view will vary depending oti the size of the image sensor (CCD).

Manual iris
These letises are manually set by the installer to obtain optimum results and are best suited to conditions where the lighting remains niore or less constant. if the camera has an electronic shutter you can use a manual iris lens in varying light providing it is not for use in day/night extremes.

Auto iris

There are two types of auto iris the video drive and DC or direct drive. The camera`s image sensor produces a video signal its strength dependent on the amount of light it receives. in video drive lenses the camera transmits this video signal to the autoiris lens where a small electronic circuit coverts it to a voltage and opens or closes the iris automatically. in DC or direct drive lenses the circuitry which converts the video signal to a voltage is contained within the camera. AUto iris lenses can be used in applications with varying lighting conditions. For use in dav/night extremes it is necessary to check the suitability of the aperture range of the lens.

Zooms
A zoom lens can vary its focal length to show a wide angled view of a whole building for Vexample or a telephoto view of one specific part of it. Zoom ratios are often easily confused. The zoom ratio is purely and simply an index to express the relationship between ilic extrenies of focal length. For example a 7.575mm lens has a 10: 1 zoom ratio and so does a 16160mm. Both would be specified to cope with different ranges of performance and it is only when the actual focal length is known that an accurate indication of the size of the image on the display monitor is given. One of the most critical factors when installing a zoom lens is the back focus adjustment of the camera the position of the image sensor. A zoom lens should remain focused throughout its focal range but if the back focus is not set correctly the image is likely to go out of focus as the lens zooms.

Varifocals
An `almost` zoom! The critical difference between a true zoom and a varifocal is that Ytinlike the zoom lens the varifocal will require refocusing if the focal length is changed. These lenses generally have a short focal range and are useful in situations where the desired image falls between two standard lenses.

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