Sample Chapter: 4 Lenses
From CCTV Information
Sample chapter from Mike Constant's book: The Principles & Practice of CCTV
Lenses
The human eye is an incredibly adaptable device that can focus on distant objects and immediately refocus 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 simultaneously. 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, select the appropriate angle of view and refocus. 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 plane becoming progressively out of focus. The angle of view is fixed. At any 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 be 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 much 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. The important scene can be 'whited out' or too dark to be of any use.
A controversial but important aspect of designing a successful CCTV system is the correct selection of the 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 it through their own eyes. Topics such as identification of miscreants or numberplates 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 with one camera. The solution may be more cameras or viewing just a restricted area of particular interest. A Company putting forward the system proposal should have no hesitation in 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 be a large variation in the quality between different makes and this should be considered 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 CCTV Lens
Exposure Control
The exposure in a normal photographic camera can be controlled by a combination of shutter speed and iris opening. This is not so with a CCTV camera lens. A standard CCTV camera produces a complete picture every 1/2 of the mains frequency. This is every 1/25 second where the mains frequency is 50 Hz (cycles per second) and every 1/30 second where the mains frequency is 60 Hz. Generally the exposure time is fixed and the only control of the amount of light passing to the imaging device is by adjusting the size of the iris. This is covered in more detail later in this chapter. Most camera tubes and imaging devices have some tolerance of the amount of light passed by the lens to create an acceptable picture. The range of tolerance is generally inversely proportional to the sensitivity of the camera. The more sensitive cameras require greater control of the iris aperture.
Types of Lenses
Lens format
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. This is a particular design of thread size and flange length originally used on photographic cameras. In recent years lenses have been developed for the 2/3", 1/2" and now 1/3" format cameras. Consequently, great care must be exercised when selecting a lens for a particular camera. Just as there are four formats of camera so there are four formats 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. The difference between the two types of mount is the flange back length, which is the distance from the back flange of the lens to the face of the sensor. See diagram 4.1. The screw thread and shoulder length for each type of mount is identical. This makes it impossible to see the difference except that the overall size of the CS-mount lens is generally smaller.
A C-mount lens may be used on a CS-mount camera with an adapter ring but a CS-mount lens cannot be used on a C-mount camera. The main problem is that either type of lens can be screwed onto both types of camera without apparent damage. The result is that if the wrong type is used it will be impossible to focus the camera. Some C-Mount lenses have a projection at the back that could damage the sensor in a CS-Mount camera.
Diagram 4. 1 Types of Lens Mounts
A chart is provided at the end of this chapter showing the relationships between different lenses and camera combinations and the associated angle of view. At the time of going to press, most 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. Therefore, there would be a significant variation in the expected scene content if this fact were overlooked.
| FORMAT | 1" | 2/3" | 1/2" | 1/3" |
| ANGLE OF VIEW | 29° | 9.5° | 114° | 9.79° |