Converting CCTV lens from video auto iris to DC auto iris

 

Converting CCTV lens from video auto iris to DC auto iris

Summary

It seems relatively easy to convert a video auto iris CCTV lens to DC auto iris and for a modern camera to control the lens correctly. I was able to convert a sophisticated expensive CCTV lens to DC auto iris, which otherwise was unusable. 

 

However, I give no guarantee that it will work with any other lens, although I think the principle is the same.

 

If camera control is not possible or desired, it is possible to at least open the lens's iris with a voltage through a series resistor applied to the drive motor with the correct polarity.

The problem

I had bought a Pentax motorised zoom and focus lens for use in my amateur TV studio with the idea of using it on a remote-controlled tripod as part of a one person operation. Motorised lens are not cheap ($600), but I bought a new, but old stock, lens cheap (<$100).

 

The main problem was that the lens used video auto iris, rather than DC auto iris, that is the standard on digital CCTV cameras. If not used, the video auto iris closes the lens, so at a minimum I needed to open the iris to use the lens. DV CCTV cameras can use lenses with an open iris or no iris or a manual iris.

 

I am a complete novice with CCTV, just learning as I go ( and over-whelmingly impressed with what is possible with digital CCTV now). However, I am a radio amateur, VK4ZXI, and a graduated engineer with some knowledge of electronics.

How auto iris works

Lens can use video auto iris (common with old analogue lenses), DC auto iris (common for most modern lenses) or no iris (cheap lenses).

 

The connection diagram for my lens shows the video auto iris and the two control coils for the auto iris, a motor drive and a "galvanic" coil. The motor drive proportionally opens and closes the lens. The galvanic coil measures the rate of change and is used as feedback for the control system for the lens.

 

 

For video auto iris, a video signal from the image sensor (I don't know what format) is feed to the lens and some control circuitry (EE AMP) generates the required signals for the motor and galvanic coils.

 

For DC auto iris, there are just the four wires for the motor and galvanic coils. The camera does the iris control instead of it being in the lens.

 

With some trepidation, I removed the lens cover and removed the EE AMP circuit board, leaving just the four wires for the two windings. Fortunately I had a cheap DC auto iris lens that I could dismantle and to salvage the auto iris connector that suited my HD CCTV camera.

 

I measured the resistance of the coils on both lens. The lower resistance I presumed was the motor coil and the higher one, the galvanic coil.

 

I then applied a variable voltage from a power supply, though a series resistor to limit current (1000 Ohm I think). It was possible to open the lenses with either coil, but polarity was important on both (I discovered, but not surprising). The iris would open with about 5 V DC at less than 10 mA. Eventually I had identified the polarity of the coils and which was the drive motor and which was the galvanic coil, per my diagram below. The left diagram is measured at the connector for the cheap lens, the right is for the Pentax lens.

 

Measuring on the connector gave me the connections to the camera, some vital information. As can be seen, I originally misinterpreted which was the drive motor and which was the galvanic coil.

 

 

 The next step was to see if the camera could control the lens. I cut off the connector from the cheap lens and connected it to the lens with jumper leads per my connections above. With some trepidation of blowing up both, I turned it on and it worked perfectly!

 

 

The Pentax lens is attached to a HD CCTV camera. The composite output is fed to a 7" focus screen (HDMI input not SDI). The cheap lens is at bottom left and not connected to anything. The EE AMP green circuit board is to the bottom right of the cheap lens and not connected. The black connector, centre bottom, is for the motorised focus/zoom and has wires to control them (5 - 12 V DC), which works. The image is of the kitchen range hood and a box of "ALL-Bran" in the cupboard next to the range hood. The camera is not easy to move with all the jumper leads connected! The minimum focus distance of the lens is about 2 m, with the range hood about 5 m distant. There is glare on the monitor from a screen door.

 

The photo was taken during the day with the lights on. At night, with the kitchen lights off, the monitor image was better than visual, indicating that the auto iris was working well.

 

As a side note, the iris settings can be changed with the camera menu, allowing creative control over depth of field etc, if used as a cheap full HD SDI cinema camera with remote recording or live.

Conclusion

The bottom line is that it is relatively easy to convert a video auto iris lens to DC auto iris and for a modern camera to control the lens correctly. (the opposite is difficult) I was able to convert a sophisticated expensive CCTV lens to DC auto iris, which otherwise was unusable. 

 

However, I give no guarantee that it will work with any other lens, although I think the principle is the same.

 

If camera control is not possible or desired, it is possible to at least open the lens's iris with a voltage through a series resistor applied to the drive motor with the correct polarity.