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Guide to Closed Circuit TV (CCTV)

TRUSTED - Target Recognition Using Surveillance Technology for Evidence and Detection - A campaign to improve the effectiveness of existing video surveillance security systems.

MANUAL LENSES for Industrial CCTV Cameras (continued)

The first thing to say about manual zoom lenses, is that as they are quite complex optical devices, and indeed fairly expensive, there really aren't that many of them. The first important point about zoom lenses (and indeed varifocals) is the concept of ‘zoom ratio’. It’s actually quite a simple principle, but more often than not thoroughly misunderstood by many.

If we consider for example an 1/2” format 8 - 48mm lens, this would be described as a 6:1 (pronounced ‘six to one’) zoom, as the longest focal length at 48mm, is six times greater than the shortest at 8mm.

If you think about the range of focal lengths in just this one lens, it is quite obviously capable of offering unique benefits under certain key conditions; after all, to provide an equivalent level of flexibility in setting up a particular view, you'd probably need to have half a dozen fixed lenses, just to cover the same range (i.e. 8mm, 12mm, 16mm, 25mm, 35mm, 50mm).

Anyway, coming back to the subject of zoom ratio, this is where things can begin to get a little sticky. The most common mistake with zoom lenses (and varifocals) is to try and relate the zoom ratio to the actual lens magnification.
The example 8 - 48mm is by definition a 6:1 zoom ratio, but this does not actually provide 6x magnification ..... why’s that then?

Well, on a 1/2” CCD camera, the standard focal length lens would be a 12mm (magnification is x1, and roughly equivalent to the same angle of coverage provided by the human eye);

With this in mind, our 6:1 optic would actually be equivalent to a maximum magnification of 4x (12mm x 4 = 48!). Now you cannot use a 1/2” lens on a larger format camera (such as 2/3”), but there’s nothing to stop you using it on a smaller format, like a 1/3” CCD.

On this format, standard focal length is 8mm, so the maximum magnification is 6x (8mm x 6 = 48) and coincidentally, the zoom ratio is 6:1.

If you think this is complicated, just wait for the technical stuff!!


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