CCTV Camera C/CS Lens

 Brooke Clarke



Background

When working with C/CS type CCTV cameras you need a lens.  My application is two fold.  One is to have a web cam, although now limited to still images at a slow rate, and the other is astronomical.

Theory

Lens Specifications

Focal Length (FL)

This is the distance between the optical center and the image when the object is at infinity.  When the object is twice the FL in front of the lens the image is twice the FL behind the lens.  You can not focus on an object that's closer than twice the FL from the optical center.

The FL determines the scale factor, magnification, field of view, etc.  This comes about from simple trignometry using a line on the central axis of the lens and another line at the edge of the field of view.  Note that this gemotery changes depending on how far away an object is from the lens. 
For objects very far away:
Field Of View (deg) = 2 * ATAN( Is / (2 * FL))
where:
Is = the image size which depends on the size of imaging chip in the camera in the same units as the FL, commonly mm.

Field of View table showing the effect of focal length on various imaging chip sizes


1/4 " 1/3 " 1/2 " 2/3 " 1 " 35 mm Film
FL mm
3.6 2.7 4.8 3.6 6.4 4.8 8.8 6.6 12.8 9.6 36 24

H
V
H
V
H
V
H
V
H
V
H
V
1.6 97 80 113 97 127 113 140 128 152 143 170165
2.5 72 57 88 72 104 88 121 106 137 125 164156
3.9 50 38 63 50 79 63 97 80 117 102 156144
6 33 25 44 33 56 44 73 58 94 77 143127
9.4 22 16 29 22 38 29 50 39 68 54 125104
15 14 10 18 14 24 18 33 25 46 35 10077
23 9 7 12 9 16 12 22 16 31 24 7655
35 6 4 8 6 10 8 14 11 21 16 5438
55 4 3 5 4 7 5 9 7 13 10 3625
85 2 2 3 2 4 3 6 4 9 6 2416

Using 35 mm Film Camera Lens on CCTV camera

There is an advantage in using a 35 mm film camera lens in that the objective diameter is much larger than a CCTV lens.  Remember that for stars the f-number does not mean much what's important is the area that's gathering the light.  For example a 6 to 60 mm varifocal CCTV lens has an objective diameter of 16mm and a 50 mm film camera lens rated f/1.4 would have an objective diameter of 36 mm.  That's about 5 times the area for the film camera lens.

Iris (Diaphram)

This is an issure for a security camera or a web camera that will be looking at things on the Earth.  For looking at start the iris will typically always be wide open.

A manual iris lens is easier to make than an automatic iris lens and so costs less.  But if the scene is changing brightness the auto iris lens is far superior.  There are two common types of control for the auto iris lens.  The older is called video drive where the lens contains electronics to to convert a video signal into a control signal.  These typically have one or two pots for adjusting the level and optionally the speed of operation.

The more modern type is the DC drive where the camera converts the video signal into a drive current.  In this case the camera may have a pot to set the level.  The 4 pin connector has labels like:
Pin #
Description
Name
Name
Name
1

Dump - Control - Damper Lo
2

Dump + Control + Damper Hi
3
Galvo motor drive1
Drive Drive + Drive
4
Galvo motor drive1
Ground Drive - Ground
Note 1 = By applying  5 ma to pins 3 & 4 with 3 positive, when the current gets to about 7 ma the iris goes from fully closed to fully open.

Iris Dynamic Range

The first generation auto iris was made by adding electromagnetic control to a camera type iris.  The range might be 8 stops (i.e. f/1.4 to f/22) but these don't work too well with the starlight type security cameras because at noon they can not stop down far enough.  The new auto iris mechanism uses neutral density filters either instead of or in addition to the manual iris (not sure which) to double the range, for example from f/1.4 to f/360.

Zoom Wide Dynamic Range but not IR type:

Exposure Priority Modes

In Shutter (Sense?) Priority the iris starts out at the highest f/# (say f/360) and the electronic shutter is used to control the exposure.  Once the shutter is at the longest possible exposure, say X128 sense up or 2 seconds on the Mintron,  then the iris starts to open up.  This mode is great for most things where there's not much movement in a scene.

In iris (AGC?) Priority the electronic shutter starts out at the fastest possible speed (say 1/1000 second) and the iris is used to control the exposure.  Once the iris is wide open (say f/1.4) then the shutter speed starts slowing down.  This is good for things like traffic cameras where it's important to freeze motion blur, but does not provide as good an image as shutter priority.

Switch Settings

AES Automatic Electronic Shutter

This has to do with getting the image at the sensor chip within it's dynamic range.
When ON enables the electronic shutter.  This is the setting used when a non auto iris lens is used.  Turn off when auto iris lens is controling the exposure.
I'm guessing that if an auto iris lens is used with it set to ON the exposure may become unstable, i.e. jumping back and forth between too bright and too dim.
There is a time lag of a few seconds.  If you open the manual iris the AES will make a shorter exposure, this can continue until the electronic shutter is at the fastest speed then the image will wash out.   Going the other way as you stop down the iris the shutter will stay open longer, but the max time is one frame.  Once at one frame the AES does not start frame integration on cameras like the P-38.

AGC Automatic Gain Control

This has to do with the voltage of the video signal coming out of the camera.
When ON the gain of the output amplifier is adjusted to bring the average luminance (brightness) of the scene to a standard gray level.  This is very similar to the brightness control on a TV set.

The Mintorn cameras have three settings for AGC.

Day & Night IR Lens

Rainbow is starting to introduce a new lens design that they are branding as Day-Night-IR.  In addition to the wide dynamic range iris control (see above) these also have an optical design that eliminated the focus shift normally encountered when working with near IR as comparte to visible light.  They way the lens stays in focus as a "Day-Night" camera switches from visible light daytime mode to IR illuminator nightime mode.

Since the light from satellites and meteors contains a lot of near IR these lenses would also be good for imaging them.

28 Sep 2007  Varifocal IR Type:

Motorized Zoom IR Type:

Links

Vendors

123 Security Products - seem to have the most complete stock of Rainbow lens

Rainbow - not a sales outlet, but has on line data sheets and the more complete manual listing even more products.

Back to Brooke's  PRC68, Products for Sale, Photography, Binoculars, Astronomy, CCD AstronomyElectro-optical Gadgets, Military Information, Personal Home page

page created 10 Oct 2007.