Welcome to the
"Technical Section"
of
Doktor Jons
Guide to Closed Circuit TV (CCTV)

All you need to know about lenses,
for all types of video surveillance.

PHOTOGRAPHIC LENSES for Industrial CCTV Cameras (cont.)

Through the clever use of mirrors (thats optical not vanity!), the light rays are brought into focus, providing a very long focal length within a relatively compact package.

For example, a 600mm lens (that’s 75x magnification on 1/3” CCD) is little more than 4” or 100mm in length.

The down side to using these cats, is their relatively slow speed; with maximum apertures often in the region of f8, you have to use an extremely sensitive camera to get the best out of them. For the more extreme Big Cats, at 1000mm or even 2000mm, it may be worth considering working purely with a monochrome camera, possibly filtered just for Infra Red imaging.

Atmospheric pollution can seriously degrade an image over a distance of hundreds of metres, but if you fit an Infra Red pass filter, this tends to cut through much of the mist and smog. Should you ever wish to try a 2000mm lens on for size, you can expect to fill the screen with a newspaper at around 500metres from the camera!

The most obvious problem with using these extreme telephoto lenses, is that of image stability.

As soon as you set up the camera even on the sturdiest of supports, supports, something as simple as a passing vehicle, or someone walking across the floor can set up enough ground movement to render the images unwatchable.

In practice, pretty well any telephoto up to 500mm can be used very successfully, and in general terms, those around 2 - 300mm offer the best compromise for ‘pulling power’ against light gathering capability, normally with maximum apertures of around f2.8 - f4, and using high dispersion (ED) glass, they most certainly do not come cheap!

Be aware though that at these extreme focal lengths, depth of field can be incredibly short, and so accurate focussing on a subject is absolutely critical.

Let's get closer, with Macro Lenses>

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