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.

MANUAL IRIS LENSES for Industrial CCTV Cameras (continued)

Improving performance in the peripheral areas is known technically as ‘edge enhancement’, and whilst lens designers spend an awful lot of time and money trying to produce optics with excellent centre and edge definition, many aspects of the eventual design can become something of a trade off.

After all, if you make the best telephoto CCTV lens in the world which is ‘optically corrected’ beyond our wildest dreams, not many will buy it if it costs three times more than the camera. So the bottom line is that most designs suffer from the dreaded ‘c’ word ... compromise.

In practice, whilst the edge resolution of a lens will always be inferior to the centre, there is a very simple technique which can make a radical difference to the overall performance ... closing the iris!

Yup, it’s true, if you twist the aperture ring, or ‘stop down’ the lens as we prefer to call it, both the centre and edge definition will dramatically improve .... but only to a point.

Whilst all lens designs are different, the general rule is that if you close the aperture (iris) to around its mid point (i.e. f5.6 ish) this will produce the best overall optical performance from the lens.

If you continue to close the aperture, performance will generally deteriorate, until by the time it reaches the minimum aperture (normally f22) the lens quality is probably worse than it was at the maximum aperture (perhaps f1.4).

There are some very expensive pieces of test equipment used for carrying out this laboratory testing, but whereas many manufacturers make the test results readily available for photographic lenses, in the CCTV world it’s almost unheard of.

To be fair, many ‘quality’ CCTV lenses will far outperform even the best of the analogue CCTV cameras on the market (Mega-Pixel cameras require special higher resolution lenses), and whilst most indoor situations, will generally only allow the lens to be set to around f4 or f5.6, this would actually provide the best quality that the lens can produce. It’s also worth remebering that at f4, most cameras fitted with ‘Electronic Iris’ will work quite happily outdoors, during the day.

Automatic Iris Lenses >

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