Technical notes
Autoiris lenses

Sito in italianoAutoiris lens 8mm f1.3 for 2/3" servomotor type


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There are two types of currently available autoiris lenses: ring (otherwise called servomotor) type and galvano type. The latter can be with built-in amplifier (so called video drive) or without amplifier (DC drive).

The servomotor type operates open/close iris when electric power is applied. The open/close position of the blades depends on camera video signal. Iris operation and fine adjustment is quite accurate and stable and is depending on a magnetic device. This type of autoiris lens is less sensitive to vibration and its life is longer; therefore is more expensive than galvano type.

The construction of galvano type is simpler and the iris operation is rougher than the ring-servo type and it tends to be influenced by very small error. It is nevertheless a cheaper lens and this is the reason why this type is rather popular, nowadays.

The galvano autoiris lens is available in two different versions: with and without amplifier (so called video drive and DC drive).

Video drive lens operates by the camera video output signal. It can be easily recognized because , comparing with DC drive, it has a 3 wires connection (video, power, ground). Its operation can be controlled by ALC and level potentiometers. The connector to the camera is normally not supplied with lens, because there is no internationally recognized standard on it and every single camera manufacturer can have a different standard.

DC Drive lens operates with 2 coils: driving coil is to move the open/close iris operation while damping coil is used to break iris movement (consider it as a reaction to driving coil operation), thus controlling  the stability of iris movement. This lens has a 4 wires connection. Connector can be supplied with lens being a standard on most cameras. This lens is cheaper than the video drive type, lacking the built-in amplifier which is incorporated into the camera.

Most of recent cameras now offer a video/DC switch in order to be able to use both types of lenses.

How to set an auto iris lens

Auto iris lenses have two potentiometers ALC and Level. In DC driven lenses the settings will be found on camera itself.

Both settings  are adjusted by the manufacturer at a default value which is good for most applications and thus no further adjustment is usually needed.

ALC (Automatic Light Compensation) is a photometric adjustment of the iris and can be considered similar to the backlight compensation usually found in a camera. It sets the signal reference level according to the contrast of the image and it has two ends: Peak e Average. Very bright points on the image make the iris closing and any dark object will be even darker and sometimes too dark to distinguish any detail. In this case we may adjust the ALC in order to make the iris open more than usual (setting towards the Peak position). On the contrary when it is necessary to see better the bright points the ALC will be set towards the Average position in order to make the lens closing more than usual. It should be noted that the ALC setting will be effective onlty with high contrast scenes.

Level  is a sensitivity adjustment to the video signal changes and shows two end positions: H (high) and L (low). By turning the screw towards H the image will be brighter and consequently will be darker when turning the screw towards H.
Setting the Level you should take care that the image is acceptable both in daylight and in night operations.

Both settings should be made always with AGC=Off.

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Last modified 10-05-2013
 
 
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