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.

AUTO IRIS LENSES for Industrial CCTV Cameras

We’ve looked at the various benefits of using manual iris lenses for almost all indoor applications, and yet the vast majority of surveillance cameras are still installed with automatic iris CCTV lenses.
Does Doktor Jon think this is a good thing .... well, absolutely not!

Auto Iris or AI as they used to be more commonly known, are pro rata far more expensive, more complicated, and generally inferior in terms of overall optical quality, than their manual iris equivalents.
With most indoor applications, the lighting levels rarely change to the degree they would if the camera were being used outdoors.

Wherever people are present, they need some degree of lighting to help them see where they’re going, and if a human eye can see, then a CCTV camera can also see, so why blow all that extra money on something you don’t necessarily need or want.

Doktor Jon is not happy with the current thinking ... because there is a nice picture produced with an AI lens, that’s the way it oughta be. The guy up the road has AI lenses, so we must also have the same.

There are essentially two main types of auto iris lens:-

The oldest and most complex are the ‘video amp’ or ‘video iris’ type. These have an internal printed circuit board which controls the lens iris to maintain the correct level of highlights (overall exposure) in the image.

This is achieved by taking a power supply and video signal directly from the camera, through a thin ‘plugable’ multicore cable. Inside the lens itself, there are normally two externally adjustable ‘pots’ (potentiometers or variable resistors) which are used to set up the lens operation. One is marked as ‘Level’ and this sets the threshold for when the lens iris starts to operate. The other is marked ‘ALC’, and this one is supposed to control exposure (i.e. the amount of light that passes through the lens).

In practice, most lenses are left with the’ALC’ setting fully anti-clockwise, and the ‘Level’ pot turned to just past the point where the iris begins to function automatically, for the correct overall picture quality.

Automatic Iris Lenses Continued>

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