Cameras and Lenses Made Simple
The subject of specifying cameras is a jungle of jargon and misinformation, this brief article attempts to shed a little light on some of the mysteries surrounding it. Only CCD cameras will be considered because they are now the most commonly used type for CCTV.
The imaging device:
CCD means a Charged Coupled Device and consists of a flat array of tiny, light sensitive photodiodes. Each diode produces a voltage that is directly proportional to the amount of light falling on it. No light would produce no voltage and therefore a black level. Maximum light would produce a maximum voltage and therefore a white level. In between these would be shades of grey, and is the luminance information of a video signal. In the case of a colour camera, a chrominance signal is superimposed onto the luminance signal to carry the colour information. (If a colour camera is connected to a monochrome monitor, then a monochrome picture would be produced from the luminance information and the chrominance would not be processed). See also colour cameras with separate Y/C outputs under resolution.
The range of light levels that a CCD can cope with is very limited, therefore means have to be introduced to restrict the light range within certain limits.
The video signal:
A field of video is created by the CCD being scanned across and down exactly 312 1/2 times and this reproduced on the monitor. A second scan of 312 1/2 lines is exactly 1/2 a line down and interlaced with the first scan to form a picture with 625 lines. This is known as a 2:1 interlaced picture. The combined 625 line is known as a frame of video and made up from two interlaced fields. The total voltage produced is one volt from the bottom of the sync pulse to the top of the white level, hence one volt peak to peak(p/p). The luminance element of the signal is from 0.3 volts to one volt, therefore is 0.7 volts maximum. This is known as a composite video signal because the synchronising and video information are combined into a single signal.
Note that the imaging device is scanned 625 times but the actual resolution is defined by the number of pixels making up the device.
There are several factors that make up a complete camera specification and are all be inter-related. These are:
Signal to noise ratio.
Automatic gain control.
The most common factor people look for in a camera specification is the sensitivity, although it is not always the most important. Sensitivity is the amount of light, in lux, necessary to produce a video signal of some, usually unspecified, level. This factor seems to be the marketing battleground upon which all manufacturers fight to show their cameras as being better than the competition!
Signal to noise ratio. (S/n).
As seems obvious this is the ratio of the level of the video signal to the amount of noise present. Noise in a video is seen as snow or graininess, resulting in a poorly defined image on the monitor or video recording. The unit for expressing s/n ratio is decibels (dB), but do not be too worried because it can be expressed as a ratio. The following table shows the equivalent ratio for values given in dB.
It can be seen that a s/n ratio of 40Db is equivalent to a ratio of 100:1, that is the signal is 100 times the noise level. Conversely the noise is one hundredth of the signal. Note that at a s/n ratio of 20Db, the noise is 10% of the signal and would produce an unacceptable picture. The following table provides a guide as what quality to expect from various s/n ratios.
|S/N ratio dB||S/N ratio:1||Picture quality|
|60 dB||1,000||Excellent, no noise apparent|
|50 dB||316||Good, a small amount of noise but picture quality good.|
|40dB||100||Reasonable, fine grain or snow in the picture, fine detail lost.|
|30 dB||32||Poor picture with a great deal of noise.|
|20 dB||10||Unusable picture.|
Automatic gain control (AGC).
When the light falling on to an imaging device reduces to a certain level, there is insufficient to create a full level video signal. AGC acts to increase the amount of amplification in these conditions to bring the signal up to the required level. As well as amplifying the video signal, additional noise can be introduced, and the signal to noise ratio reduced. The result is frequently a very much degraded signal and poor picture on the monitor.
The value referred to here is the horizontal resolution in TV lines, that is the number of black to white transitions that can be resolved across the image. This is a function of the number of pixels that make up the CCD imaging area and the bandwidth of the camera circuitry. Typical camera resolution is 350 TV lines, with high resolution cameras producing better than 450 lines. Note that resolution costs money!
There are now colour cameras that instead of superimposing the chrominance onto the luminance signal, provide the chrominance as a separate signal. This is known as Y/C separation and requires two coaxial cables from the camera to carry each signal separately. The effect of this technique is to increase the bandwidth and therefore the resolution, typically to better than 500 TV lines.
The human eye is an incredibly adaptable device that can focus on distant objects and immediately re-focus on something close by. It can look into the distance or at a wide angle nearby. It can see in bright light or at dusk adjusting automatically as it does so. It also has a long 'depth of field' therefore scenes over a long distance can be in focus at the same time. It sees colour when there is sufficient light but switches to monochrome vision when there is not. It is also connected to a brain that has a faster updating and retentive memory than any computer. Therefore the eyes can swivel from side to side and up and down, retaining a clear picture of what was scanned. The brain accepts all the data and makes an immediate decision to move to a particular image of interest. It can then select the appropriate angle of view and re-focus. The eye has another clever trick in that it can view a scene of great contrast and adjust only to the part of it that is of interest.
By contrast the basic lens of a CCTV camera is an exceptionally crude device. It can only be focused on a single plane, everything before and after this becomes progressively out of focus. The angle of view is fixed at any one time it can only view a specific area that must be predetermined. The iris opening is fixed for a particular scene and is only responsive to global changes in light levels. Even an automatic iris lens can only be set for the overall light level although there are compensations for different contrasts within a scene. Another problem is that a lens may be set to see into specific areas of interest when there is a lot of contrast between these and the surrounding areas. However as the sun and seasons change so do light areas become dark and dark areas become light so the important scene can be 'whited out' or too dark to be of any use.
One of the most controversial but important aspects of designing a successful CCTV system is the correct selection of lens. The problem is that the customer may have a totally different perspective of what a lens can see compared to the reality. This is because most people perceive what they want to view as they see through their own eyes. Topics such as identification of miscreants or number plates must be subjects debated frequently between installing companies and customers.
The selection of the most appropriate lens for each camera must frequently be a compromise between the absolute requirements of the user and the practical use of the system. It is just not possible to see the whole of a large loading bay and read all the vehicle number plates. The solution may be more cameras or viewing just a restricted area of particular interest. The company putting forward the system proposal should have no hesitation of pointing out the restrictions that may be incurred according to the combination of lens versus the number of cameras. Better this than an unhappy customer who is reluctant to pay the invoice.
Fixed Focal Length
These are sometimes referred to as monofocal lens. As the name implies this type of lens is specified when the precise field of view is fixed and will not need to be varied when using the system. The angle of view can be obtained from the supplier's specification or charts provided. They are generally available in focal lengths from 3.7mm to 75mm. Longer focal lengths may be produced by adding a 2x adapter between the lens and the camera. It should be noted that this will increase the f number by a factor of two (reducing the amount of light reaching the camera). If focal lengths longer than these are required then it will be necessary to use a zoom lens and set it accordingly.
Except for very wide angle lenses all other lenses have a ring for adjusting the focus. In addition cameras include a focusing adjustment that moves the imaging device mechanically relative to the lens position. This is to allow for minor variations in the back focal length of lens and manufacturing tolerances in assembling the device in the camera. Correct focusing requires setting of both these adjustments. The procedure is to decide the plane of the scene on which the best focus is required and then set the lens focusing ring to the mid position. Then set the camera mechanical adjustment for maximum clarity. Final fine focusing can be carried out using the lens ring.
The mechanical focusing on cameras is often referred to as the back focus. This was because a screw at the back of the camera moved the tube on a rack mechanism. Modern cameras now have many forms of mechanical adjustment. Some have screws on the side or the top, some still at the back. There are cameras that have a combined C/CS-mount on the front that also has the mechanical adjustment and can accept either type of lens format. The longer the focal length of the lens the more critical is the focusing. This is a function of depth of field.
Variable Focal Length
This is a design of lens that has a limited range of manual focal length adjustment. It is strictly not a zoom lens because it has quite a short focal length. They are usually used in internal situations where a more precise adjustment of the scene in view is required which may fall between two standard lenses. They are also useful where for a small extra cost one lens may be specified for all the cameras in a system. This saves a lot of installation time and the cost of return visits to change lenses if the views are not quite right. For companies involved in many small to medium sized internal installations such as retail shops and offices this can save on stockholding. It makes the standardisation of systems and costing much easier.
Manual Zoom Lens
A zoom lens is one in which the focal length can be varied manually over a range by means of a knurled ring on the lens body. It has the connotation of 'zooming in' and therefore infers a lens with a longer than normal focal length. The zoom ratio is stated as being for instance 6:1 this means that the longest focal length is six times that of the shortest. The usual way of describing a zoom lens is by the format size, zoom ratio and the shortest and longest focal lengths, i.e. 2/3," 6:1, 12.5mm to 75mm. Again, great care must be taken in establishing both the camera and the lens format. The lens just described would have those focal lengths on a 2/3" camera but a range of 8mm to 48mm on a 1/2" camera. Similarly a lens giving the same performance on a 1/2" camera would be a 1/2," 6:1, 8mm to 48mm.
Motorised Zoom Lens
Manual zoom lenses are not widely used in CCTV systems because the angle of tilt of the camera often needs to be changed as the lens is zoomed in and out. The most common need for a zoom lens is when used with a pan tilt unit. The lens zoom ring is driven by tiny DC motors and controlled from a remote source. With a correctly set up camera lens combination the focus should not change from one limit of zoom to the other.
With the development of ever smaller cameras and longer focal length lenses the method of mounting the camera/lens combination must be taken into account. There are many cases where the lens is considerably larger than the camera and it may be necessary to mount the lens rigidly with the camera supported by it. In other cases it may be necessary to provide rigid supports for both camera and the lens. Always check the relationship between the camera and lens sizes and weights when selecting a housing or mounting. Most manufacturers of housings can provide lens supports as an accessory.
The most frequent reason for the focus changing when zooming is that the mechanical focus of the camera has not been set correctly.
Motorised Zoom Lens With Pre-sets
There are many situations where it is required to pan tilt and zoom to a predetermined position within the area being covered. It is possible to obtain motorised lenses with potentiometers fitted to the zoom and focusing mechanisms. These cause the lens to zoom automatically and focus to the setting by measuring the voltage across the potentiometer and comparing it with the signals in the control system. All other functions are as for motorised zoom lenses. Pre-set controls are only possible with telemetry controlled systems. The specification of the telemetry controls should be checked to see whether the pre-set positions are set from the central controller or locally from the telemetry receiver.
This chapter is supplied by Mike Constant and was originally published in CCTV Today. Mike is the author of 'The Principles & Practice of CCTV' which is generally accepted as the benchmark for CCTV installation in the UK.