Selection principle of surveillance camera lens

With the development of China's smart building industry, the position and role of security technology in smart buildings are also increasing. As a security technology, closed-circuit monitoring television (CCTV) system is playing an increasingly important role in star-rated hotels, foreign-related office buildings, banks, government agencies and other places, including the current smart community also uses CCTV system as security And an important means of property management. In order to make CCTV technology play a better role in intelligent buildings, in each CCTV monitoring project, how to choose the camera lens correctly is very important for economic indicators and technical performance.

Lens classification
The types of lenses can be roughly divided into:

(1) Wide-angle lens: The angle of view is above 90 degrees, and it is generally used in places with small line-of-sight and large angle of view, such as in elevator cars and halls;
(2) Standard lens: the angle of view is about 30 degrees, and it is generally used in places such as walkways and perimeters of the community;
(3) Telephoto lens: the angle of view is within 20 degrees, and the focal length ranges from tens of millimeters to hundreds of millimeters, which is used for long-distance monitoring. (4) Zoom lens: the focal length of the lens is variable, from wide angle to telephoto , Used in areas with large depth of field and wide angle of view; (5) pinhole lens: used for concealed monitoring.

Lens focal length determination
When choosing a lens, the following five factors determine the lens standard:

(1) Monitor the size of the scene;
(2) The size of the object being photographed;
(3) Object distance;
(4) Focal length;
(5) CCD target size.

The first 4 points can be measured on site and calculated to determine the focal length of the lens. The calculation method is as follows:

u 1/3 ″ CCD F = 4.8 × L / W or F = 3.6 × L / H
u 1/2 ″ CCD F = 6.4 × L / W or F = 4.8 × L / H

Among them, W is the width of the subject; H is the height of the subject; L is the distance between the lens and the subject; F is the focal length of the lens.

So why consider the size of the CCD target surface in the selection of the lens? In order to obtain the same angle of view from the 1/3 "and 1/2" CCD cameras, the focal length of the 1/3 "CCD camera lens must be shortened; 3 ″ CCD and 1/2 ″ CCD cameras use the same focal length lens, what about the situation? 1/3 ″ CCD camera angle of view will be significantly reduced than 1/2 ″ CCD camera, while 1/3 ″ CCD camera The image is enlarged on the monitor than the 1/2 ″ CCD image, producing the effect of using a long focal length lens.

In addition, we should pay attention to such a principle when choosing a lens: that is, a CCD with a small target surface can use the lens of a CCD camera with a large target surface, and the reverse is not true. The reason is: if the 1/2 ″ CCD camera uses a 1/3 ″ lens, the amount of incoming light will become smaller, the color will become worse, and even the image will be missing; otherwise, the amount of incoming light will become larger and the color will become better The effect will definitely get better. Of course, combining various factors, the camera is best to choose a lens that matches it.

Choice of manual aperture and automatic aperture
The lens aperture is divided into manual and automatic. In the past, because the use of the camera was outdoors or other special occasions, the automatic iris lens was mostly used. In the current monitoring projects, due to the large number of CCTV systems used in intelligent buildings, indoor monitoring points account for a relatively high proportion. Many engineering companies also like to use automatic iris lenses when making quotations for engineering equipment. Although the automatic iris lens is more adaptable to the light changes of the monitoring point, its price is also significantly higher than that of the manual fixed focus lens with the same focal length. Most cameras now have an electronic shutter, and the indoor light source is relatively stable. Therefore, it is not necessary to use a large number of automatic iris lenses in smart building projects. On the other hand, the automatic iris lenses used in the market are divided into two. Main categories: a. Power-driven automatic iris lens; b. Video-driven automatic iris lens. The power-driven automatic iris lens controls the lens through four lines, two of which are DC12V or DC24V power supplies to drive the motor in the lens, and the other two control lines sense the illuminance of the external light source through the light-sensing point in the lens to control the aperture. The size of the video-driven automatic iris lens is controlled by three lines, one of which is the video trigger signal to start the iris and control the size of the iris, and the other two are DC12V or DC24V power line driving motor motor. Although most black and white or color cameras on the market currently have an automatic iris lens interface, except for a few that are compatible with both types of lenses, most cameras are not compatible, and only power-driven automatic iris lenses or video-driven automatic iris lenses can be used. If some cameras are damaged during use, the newly purchased camera has the problem of compatibility with the original auto iris lens. However, when the monitoring point in the project is outdoors, it is necessary to use a lens with an automatic iris, because the dynamic range of the outdoor light changes greatly, and the environmental illuminance in the summer sun reaches 50000Lx-100000Lx; the night street light is only 10Lx, The range of change is quite large. In this case, no matter whether the camera has the function of automatically adjusting the sensitivity, that is, it is impossible to adapt to such a wide range of illuminance through the electronic shutter of the camera itself, and the effect of controlling the image effect cannot be achieved.

In the TV monitoring system, it is very important to choose the camera lens correctly according to the monitored environment on the spot, because it directly affects whether the effect of the monitored face painting seen on the monitor at the end of the system after the system is composed can meet the system ’s Design requirements (in terms of screen range or image details), so the correct selection of camera lens can make the system get the optimal design and obtain good monitoring effect.

Camera lens can be divided into manual aperture lens and automatic aperture lens in terms of aperture, and can be divided into fixed focus lens and zoom lens in terms of focal length.
The following talks about how to choose the camera lens correctly according to the use environment.

1. Selection of manual and automatic iris lenses
The selection of manual and automatic iris lenses depends on whether the illuminance of the use environment is constant.

In the case of constant ambient illuminance, such as in elevator cars, closed corridors, and rooms without direct sunlight, manual iris lenses can be used, which can be set at one time according to the actual illuminance of the environment during initial installation and commissioning of the system The lens aperture size, you can get a satisfactory brightness picture.
For the situation where the environmental illuminance is constantly changing, such as halls, windows, and lobbies that change greatly with the sunshine time, automatic iris lenses must be used (must be equipped with a camera with an automatic iris lens socket), so that you can Realize the automatic adjustment of the screen brightness, and obtain a good relatively constant brightness monitoring screen.
The control signal for the automatic iris lens can be divided into DC and VIDEO control, namely DC voltage control and video signal control. This is in the selection of the type of automatic iris lens, the connection method of the camera's automatic iris lens socket, and the selection of the drive mode switch of the automatic iris lens. The three should pay attention to coordination and cooperation.

2. Selection of fixed focus and zoom lenses
The choice of fixed focus and zoom lens depends on the size of the monitored scene range and the required clarity of the monitored scene screen.

Lens specifications (lens specifications are generally divided into 1/3 ″, 1/2 ″, 2/3 ″, etc.) Under certain circumstances, the relationship between the lens focal length and the field of view of the lens is: the longer the lens focal length, the field of view of the lens The smaller the angle (see Figure 1); in the case of a fixed focal length of the lens, the relationship between the lens specification and the field of view of the lens is: the larger the lens specification, the greater the angle of view of the lens. So the relationship It can be seen that when the lens object distance is fixed, as the lens focal length becomes larger, the picture range of the monitored scene seen on the monitor at the end of the system is smaller, but the picture details are more and more clear; As the lens specification increases, the range of the monitored scene seen on the monitor at the end of the system increases, but the details of the picture become more and more blurred. Under the premise that the lens specification and lens focal length are fixed, the CS type interface lens The angle of view will be greater than that of the C-mount lens.

The angle of view of the lens can be divided into the horizontal angle of view of the image and the vertical angle of view of the image, and the horizontal angle of view of the image is greater than the vertical angle of view of the image. Generally speaking, the angle of view we refer to generally refers to the horizontal angle of view of the image of the lens .

In a narrow monitored environment, such as an elevator car, a narrow focal length wide-angle or ultra-wide-angle fixed-focus lens should be used in a small room. For example, if the lens specification is 1/2 ″, CS type interface, the lens focal length is 3.6mm or 2.6mm The angle of view of these lenses is not less than 99 ° or 127 °. For a camera with a height of about 2.5m in a narrow space, the angle of view of the lens is sufficient to cover the entire narrow and narrow monitored space. According to the actual situation on the spot, choose a manual zoom lens such as the Nissan Computar T2Z2814CS-2 lens. This lens is a 1/3 ″ CS type manual aperture lens, and its focal length is 2 times adjustable (manual focus). The focusing range is 2.8 ～ 6.0mm, and the angle of view varies from 96 ° to 47.2 °. This kind of lens is very suitable for use in a small monitored environment. When using it, it can be easily used according to actual needs The monitoring effect of the "point" or "face" of the scene.

For general zoom (times) lenses, since the minimum focal length is usually about 6.0mm, the maximum field of view of the zoom (times) lens is about 45 degrees. If this kind of lens is used for such narrow surveillance In the environment, the monitoring dead angle will inevitably increase. Although the front-end gimbal can be operated and controlled to reduce this monitoring dead angle, it will definitely increase the engineering cost of the system (the system needs to increase the front-end decoder, gimbal, protection Hood, etc.), and the complexity of system control, so in this environment, it is not appropriate to use a zoom (x) lens.

In an open monitored environment, first of all, according to the openness of the monitored environment, the user requires the clarity of the monitored scene picture seen on the monitor at the end of the system, and the center point of the monitored scene to the camera lens The straight-line distance is the reference basis. Under the premise of a certain straight-line distance and satisfying the need to cover the entire monitored scene, you should try to choose a long focal length lens, so that you can get a monitored scene with clearer details on the monitor at the end of the system Screen. In this environment, a zoom (magnification) lens (electric variable lens) can also be considered, which can be determined according to the design requirements of the system and the performance-price ratio of the system. Two points should also be considered when selecting: (1) When adjusted to the shortest focal length (seeing the panorama), it should be able to meet the requirements of covering the main monitored scene; (2) when adjusted to the longest focal length (seeing the details), it should be able to meet the requirements of observing the details of the monitored scene. Generally, in indoor warehouses, workshops, workshops and other environments, 6 or 10 times lenses are generally used to meet the requirements, while in outdoor warehouse areas, docks, squares, stations and other environments, 10 can be selected according to actual requirements. Times, 16 times or 20 times lens (generally, the larger the lens magnification, the higher the price, high-magnification zoom lens can be selected appropriately under the premise of comprehensive consideration of the system cost).

3. Theoretical calculation of correct selection of lens focal length
The angle of view of the lens that captures the scene is an extremely important parameter. The angle of view of the lens varies with the focal length of the lens and the size of the camera (the relationship between them is as described above). The focal length of the lens covering the scene can be calculated using the following formula:

(1) f = u · D / U
(2) f = h · D / H

f: lens focal length, U: actual height of the scene, H: actual width of the scene, D: measured distance from the lens to the scene, u: image height, h: image width

When the 1/2 ″ lens is selected, the image size is u = 4.8mm, h = 6.4mm. The distance from the lens to the scene D = 3500mm, the actual height of the scene is U = 2500mm (the actual width of the scene can be calculated by the following formula H = 1.333 · U, this relationship is determined by the camera viewfinder CCD film).

Substituting the above parameters into formula (1), f = 4.8 · 3500/2500 = 6.72mm can be obtained, so 6mm fixed focus lens can be used.