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 (continued)

The other type of AI lens is known as a ‘DC iris’ or ‘Direct Drive’ (DD), which performs pretty much the same function, but relies on the iris amplifier circuit having already been built into the camera, rather than the lens itself.

This type also uses a thin multicore to carry the iris drive voltages.

Generally speaking, DD lenses are manufactured down to a price, and are therefore less likely to exhibit the same level of optical performance across the range, when compared to AI lenses.

That said, they are generally a lot cheaper than video iris lenses.

Both types of optic incorporate a graduated ND (Neutral Density) filter into the optical path.

This has the effect of increasing the minimum aperture value from around a normal f22, to perhaps f90 for an average Direct Drive lens, and f300 for an equivalent AI lens.

Whilst this technique is relatively efficient for its purpose, it does however present one additional technical disadvantage.

With an extra filter introduced into the optical path, this cannot help but degrade the original quality of the lens elements.

So in practice, depending on the working aperture of the lens, it can cause a noticeable reduction in overall optical quality, when compared with an equivalent manual iris lens.

Varifocal Lenses>

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