Lenses

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.  

Although a lens is crude compared to the human eye it incorporates a high degree of technology and development. There can also be a large variation in the quality between different makes and this should be taken into account according to the needs of a particular installation. The lens is the first interface between the scene to be viewed and the eventual picture on the monitor. Therefore the quality of the system will be very much affected by the choice of lens. For general surveillance of, for instance, a small retail shop it is possible to use a lower quality lens with quite acceptable results. As the demands of the system requirement increase then the use of a premium quality lens must be considered. The difference in cost between a poor quality and a high quality lens will be a very small percentage of the total cost of a large industrial system.

The early CCTV lenses were designed for the 1" format tube camera and many of these are still available on the market. The lens screw thread on these cameras is called a C-mount. In recent years' lenses have been developed for the 2/3," 1/2" and now 1/3" format cameras. For this reason great care must be exercised when selecting a lens for a particular camera. Just as there are four sizes of camera so there are four sizes of lenses and they are not compatible in every combination. A lens designed for a larger format camera may be used on a smaller format but not the reverse. In addition the field of view will not be the same on different size cameras. There is now a further complication in that there is a new range of lenses with what is called the CS-mount. In this case a C-mount lens may be used on a CS-mount camera with an adapter ring but a CS-mount lens can not be used on a C-mount camera. The difference between the two types of mount is the back focal length (not the same as the focal length). This is an optical change to the back focal length and is not a mechanical difference to the lens. The screw thread and shoulder length for each type of mount are identical making it impossible to see the difference except that the overall size of the CS-mount lens is smaller. The main problem is that either type of lens can be screwed onto both types of camera without damage. The result is that if the wrong type is used it will be impossible to focus the camera.

Diagram L2 shows the sensor sizes to be used when calculating fields of view and angles of view.

A chart is provided at the end of this section showing the relationships between different lenses and camera combinations and the associated angle of view. At this time the majority of lenses with a focal length of 25mm and above are still designed for 1" cameras. This means that special care must be taken when using this long focal length lens on modern cameras. For instance a 25mm 1" lens provides the following approximate angles of view on the different formats. It can be seen therefore that there would be a significant variation in the expected scene content if this fact were overlooked.

There are two other main factors that must be taken into account when selecting the most appropriate lens for a particular situation the focal length and the type of iris control. Within each of these factors there other features that will also need to be considered. Lenses may be obtained with all combinations of focal length and iris control. The selection will depend on the site and system requirement.

The focal length of a lens determines the field of view at particular distances. This can either be calculated from the formulae or determined from tables provided by most lens suppliers. Most manufacturers also provide simple to use slide or rotary calculators that compute the lens focal length from the scene size and the object distance. The longer the focal length the narrower is the angle of view. Although not strictly correct, lenses with a focal length longer than 25mm are often referred to as zoom lens.

The latter can often be achieved by ensuring that the camera is positioned so that the person is moving towards it and therefore getting larger as they approach. This need for identification frequently means a compromise between this and the size of the area in view. A similar philosophy applies to identifying number plates or any other small detail in the field of view. In many cases the only solution is to carry out actual trials on site.

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.

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.

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.

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.

With this type of lens the iris opening is set manually by rotating a knurled ring on the lens body. Typically it will have a range of settings from the maximum to fully closed although the adjustment will be rather course. This type of lens is only suitable for indoor applications where the light levels remain fairly constant. It can also be used indoors with cameras having electronic shutters making a significant saving in cost. Care must be exercised in using this camera/lens combination in external applications because the camera may not have adequate control over the total light range. Also they do not have a neutral density filter to cope with extremely bright sunlight.

In most indoor situations the overall level of light will vary significantly between summer and winter due to light from windows, skylights, etc. Therefore it is often necessary to adjust the aperture two or three times a year to maintain optimum clarity of the picture.

Many years ago tubed cameras were becoming more sensitive and their use was spreading to more outdoor applications they were very limited in the range of light that could be coped with. To overcome this problem manual iris lenses were fitted with motors bolted on to the barrel to drive the iris ring. The motors were connected by way of an amplifier to the video output of the camera and this was monitored to adjust the iris ring according to the voltage of the video signal. The lower the voltage then the more the iris would be opened until the correct video voltage was achieved and the reverse when the video voltage increased. The early amplifiers suffered from the problem of being too sensitive and responding too quickly to changes in the video signal. This caused 'hunting' of the iris opening control and resulted in fluctuating contrast of the picture. To overcome this a delay circuit was introduced in the amplifier but this sometimes caused the reverse problem of the picture changing too slowly.

Modern automatic iris lenses are now completely self contained units produced by the lens manufacturer and containing very sophisticated electronics and microscopic motors. There are three main types of automatic iris lenses.

These are sometimes referred to as a servo lens. The most common type contains an amplifier and is connected to the video signal of the camera. It also is driven by a 9 volt dc voltage provided from the camera. As was mentioned previously, the voltage of the video signal is proportional to the amount of light on the imaging device. When the light level falls so does the video level. The amplifier is continuously monitoring this voltage to maintain it at 1 volt peak to peak. As the voltage changes so the iris amplifier opens or closes the iris to maintain a constant 1 volt.

The majority of cameras that provide an automatic iris drive include socket on the rear. There are three connections, +9v, 0v, video. Unfortunately there is no current standard for this connector but most cameras are packed with the appropriate plug. This can create problems if one camera is substituted for another make during maintenance or service. It can mean that the service engineer has to change the iris plug on site that is not an easy job. In recognition of this problem many cameras are now being produced with screw terminals on the rear

They are sometimes called a galvometric or galvano lens. This type of automatic iris lens is driven by a reference voltage produced by the camera. The lens contains a driving motor to open and close the lens and a damping coil to prevent hunting. In effect the camera contains the iris amplifier and operates the iris motors. These lenses have four connections, +ve drive, -ve drive, +ve damping and -ve damping. The camera specification should be checked to ensure that it contains the circuitry for this type of lens. They are usually less expensive than lenses with a built in amplifier and simpler to install but can only be used with a limited range of cameras. Again for this type of lens many cameras are being produced with screw connectors instead of a socket.

These lenses include a light sensor similar to that in a photographic camera. This measures the light levels and adjusts the iris aperture accordingly. It requires a 12 volt dc supply that may be obtained from any source. This type of lens is not very common now it was introduced for use on cameras that did not have a video and 12 volt output. The problem was that the light sensor was pre-set and not responsive to the video level therefore the correct level was not always maintained. The vast majority of cameras now provide an automatic lens connection therefore there will only be rare cases where this lens will be required.

EQUIVALENT FOCAL LENGTHS					ANGLES OF VIEW o
1"	2/3"	1/2"	1/3"	Horizontal		Vertical
360	249	181	136	2.0		1.5
280	194	141	105	2.4		2.0
210	145	105	79	3.7		2.6
180	124	90	68	4.1		3.0
150	103	75	56	4.5		3.6
140	97	70	52	5.4		3.9
105	73	53	39	6.9		5.2
100	69	50	38	7.1		5.5
75	52	38	28	9.3		7.3
50	35	25	19	14.3		10.9
30	21	15	11	23.8		18.0
25	17.3	12.6	9.0	28.5		21.6
20	13.9	10.1	7.6	35.1		26.7
16	11.1	8.1	6.0	43.2		33.1
12.5	8.5	6.3	4.7	53.6		41.5
10.0	6.9	5.0	3.8	65.0		51.1
	6.2	4.5	3.4	70.7		56.0
	5.9	4.3	3.2	73.4		58.4
	5.5	4.0	3.0	77.3		61.9

This is a useful quick reference table to find the appropriate lens for a given angle of view.

Example: It is required to achieve a horizontal angle of view of 7° using a 1/2" camera. Look down the horizontal angle of view column to find 7.1° and across to the focal length under 1/2" format. It will be seen that a 50mm focal length lens is required. This focal length lens is only currently made in the 1" format, and is the correct selection. The vertical angle of view will be 5.5° .

Focal lengths highlighted are generally available standard lens or the nearest in this list.

* Only available as a zoom lenses.

These tables are for general guidance and have been based the correct theoretical calculations for each focal length. However the actual angles and fields of view may vary between different manufacturers and focal lengths of lenses. They are more than adequate for every day use but if accurate dimensions are necessary the appropriate data should be obtained from the manufacturer.

An extra table has been added at the right that shows the height of an average person as a percentage of the screen height. This is a very useful guide in the selection of the longer focal length lens where it is required to 'see' persons. As explained earlier this requires that the image should be a minimum of 10% of the screen height.

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. Also produced by Mike is a CD-ROM titled: 'The Complete Handbook of CCTV' which is a self study course for those who wish to become proficient in CCTV design and installation. Again you can view this product and request a purchase by following the enclosed links.

"CCTV Today" is the UK’s CCTV Magazine, packed full of informative articles as well as all the latest products and industry news. It is published every 2 months by Miller Freeman. Contact us for subscription details.

For those specifying and designing CCTV systems, an indispensable tool is Lenscalc which enables the correct choice of lens for each application. Designed by Constant Consultants, you can try it out online.