Video surveillance system camera lens parameters and selection principles

The camera lens is the key component of the video surveillance system. It is equivalent to the lens structure in our eyes and is responsible for mapping the outside scene onto a flat surface. If there is no lens, the human eye can't see any object; if there is no lens, then the image output by the camera is a piece of white, and there is no clear image output. Similar to myopia or hyperopia, the cooperation between the camera and the lens has a similar phenomenon. When the imaging plane of the lens is not exactly on the CCD chip of the camera, the image becomes unclear.

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It can be seen that the role of the lens in the closed-circuit monitoring system is very important. Engineering designers and construction personnel should always deal with the lens: the designer should calculate the focal length of the lens according to the object distance and imaging size. The construction personnel often perform on-site debugging, and part of it is to adjust the lens to the best state.

First, the classification of camera lenses

Video surveillance system camera lens variety, from the focal length can be divided into short-focus lens, medium-focus lens, telephoto lens; wide-angle, standard, telephoto lens from the field of view size; structure has a fixed aperture fixed-focus lens, Manual aperture fixed focus lens, auto iris fixed focus lens, manual zoom lens, auto zoom lens, auto iris power zoom lens, electric three variable (aperture, focal length, focus variable) lens.

Second, the main parameters of the camera lens

Focal length

We know that parallel fibers converge at a focal point through a convex lens, and the focal length from the center of the lens is the focal length. Existing lenses can generally be regarded as a lenticular lens group. For a lens with a fixed focal length, that is, a fixed-focus lens; a lens with a focal length that can be adjusted to change is a zoom lens.

The size of the focal length determines the size of the field of view. The focal length is small, the field of view is large, and the range of observation is large. However, the distant objects are not clearly resolved; the focal length is large, the field of view is small, and the observation range is small. As long as the focal length is chosen properly, even objects far away can be seen clearly. Since the focal length and the angle of view are one-to-one correspondence, a certain focal length means a certain angle of view. Therefore, when selecting the focal length of the lens, it should be fully considered whether the observation details are important, or whether a large observation range is important. If you want to see the details, choose a long focal length lens; if you look at a close scene, choose a wide-angle lens with a small focal length.

The size of the focal length not only determines the size of the field of view, but also affects the depth of field of the image to some extent. The understanding of depth of field we can start from "focus", when adjusting the camera, we adjust the camera lens to make the scene of a certain distance from the camera clear imaging, called focusing. The point where the scene is located is called the focus point. Depth of field is the sum of the range of images that are clear to the front and back of the scene. It is called depth of field, meaning that the scene within this range can be clearly photographed. The depth of field is firstly related to the focal length of the lens. The lens with a long focal length has a small depth of field and a lens with a short focal length has a large depth of field. Secondly, the depth of field is related to the aperture. The smaller the aperture, the larger the depth of field; the larger the aperture, the smaller the depth of field. In general, the foreground is much darker than the depth of the back, that is, after precise focus, the scene in the front of the focus can be clearly imaged at a short distance, and the scene within a long distance behind the focus is clear. .

(1) Classification and application based on focal length

Depending on whether the focal length of the lens is variable or not, we can divide the camera lens into a fixed focus lens, a manual zoom lens and a motorized zoom lens. The choice of fixed focus and zoom lens generally depends on the characteristics and economy of the surveillance scene. For fixed-point surveillance sites, such as cultural relic booths and charging windows, it is economical to use fixed-focus lenses. After all, fixed-focus lenses are cheaper than zoom lenses.

We can't determine which position is the best focus point, such as some narrow channels or areas. The zoom lens has more flexibility, and it can be determined according to the scene situation which focal length is most suitable. As for the range of the zoom, it can be determined according to the length of the monitoring range. The commonly used specifications are 2.8 to 13 mm and 3.5 to 8 mm.

When the motorized zoom lens is used to monitor a large scene, it is usually used with a motorized lens and a pan/tilt. The advantage of the motorized lens is that the zoom range is large, you can see a wide range of situations, you can also focus on a certain detail, plus the pan/tilt can be rotated up and down, the viewing range is very large. The motorized lens has various magnifications such as 6x, 10x, 15x, and 20x. If the reference focal length is known, the variable range of the focal length of the lens can be determined. For example, a 6x motorized lens with a reference focal length of 8.5 mm, the zoom range is 8.5 to 51 mm continuously adjustable, and the field of view is 31.3 to 5.5 degrees. The control voltage of the motorized lens is generally 8V to 16V DC and the maximum current is 30mA. Therefore, when selecting the controller, the transmission cable length should be fully considered. If the distance is too far, the voltage drop caused by the line will cause the lens to be uncontrollable. The input control voltage must be increased or the video matrix host must be replaced with the decoder control.

The length of the camera lens focal length is closely related to its field of view. The angle of view of the lens can be divided into the horizontal field of view of the image and the vertical field of view of the image, and the horizontal field of view of the image is larger than the vertical field of view of the image. Generally speaking, the angle of view of the image generally refers to the horizontal angle of view of the image of the lens. .

Depending on the horizontal angle of view of the camera lens, the camera lens can be divided into a standard lens (or a medium-focus lens), a wide-angle lens (or short-focus lens), and a telephoto lens.

Standard lens: The viewing angle is about 30 degrees. In the 1/2-inch CCD camera, the standard lens focal length is set to 12mm. In the 1/3-inch CCD camera, the standard lens focal length is set to 8mm, which is generally used for walkways and community perimeters. And other places.

Wide-angle lens: The viewing angle is more than 90 degrees, and the focal length can be less than a few millimeters, providing a wide viewing angle.

Telephoto lens: The angle of view is less than 20 degrees, and the focal length can reach several meters or even tens of meters. This lens can enlarge the influence of the captured object at a long distance, but make the observation range smaller.

2. Optical coefficient

The pupil coefficient, the aperture index, is expressed in F and is measured as the ratio of the focal length f of the lens to the aperture D of the light. Each lens is marked with a maximum F value, for example 6mm / P1.4 represents a maximum aperture of 4.29 mm. The luminous flux is inversely proportional to the square of the F value. The smaller the F value, the larger the luminous flux. The scale of the aperture index sequence on the lens is 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, etc. The law is that the exposure amount of the previous calibration value is exactly 2 of the exposure value corresponding to the exposure value. Times. That is to say, the aperture of the lens is 1/1.4, 1/2, 1/2.8, 1/4, 1/5.6, 1/8, 1/11, 1/16, 1/22, the previous value is The root value of the latter value is 2 times, so the smaller the aperture index is, the larger the aperture is, and the larger the illumination on the imaging target surface is.

The camera lens aperture of the video surveillance system is divided into manual and automatic apertures. The manual aperture is suitable for occasions where the brightness does not change much. The amount of light entering is adjusted by the aperture ring on the lens, and it is adjusted once. The auto iris lens automatically adjusts the clear aperture as the light changes, ensuring that the brightness of the camera is relatively stable, and is used in outdoor and populous areas where light changes are frequent and frequent.

Auto-iris lenses are currently divided into two categories: one is called video-driven, and the lens itself contains an amplifier circuit that converts the video amplitude signal from the camera into control of the aperture motor.

The other type is called DC (DC) drive type, which uses the DC voltage on the camera to directly control the aperture. This type of lens only contains a galvanometer aperture motor, which requires an amplifier circuit inside the camera. For all types of auto iris lenses, there are usually two adjustable knobs, one is ALC adjustment (metering adjustment), there are two options for peak metering and average metering according to target lighting conditions, generally taking average metering files; The other is LEVEL adjustment (sensitivity), which makes the output image brighter or dim.

The ALC (Automatic Lens Control) adjustment on the auto iris lens is used to set the metering system to set the reference signal strength for the average brightness of the entire screen or the brightest part (peak) of the screen for automatic aperture adjustment. In general, the ALC has been set at the factory and can be adjusted. However, when a scene with a very high brightness is included in the scene, the image of the bright object may cause “white level clipping”. Make all screens white, and you can adjust ALC to change the picture.

(1) Resolution of the lens

The intrinsic indicator describing the quality of the lens image is the optical transfer function and distortion of the lens. However, for the user, only the spatial resolution of the lens needs to be understood. The number of black and white stripes that can be resolved per millimeter is the unit of measurement. The formula is: Lens resolution N two 180/frame format height. Since the camera CCD target size has been standardized, such as a 1/2 inch camera, the target surface is 6.4 mm wide by 4.8 nm high, and the 1/3 inch camera is 4.8 mm wide by 3.6 mm high. Therefore, for a 1/2-inch format CCD target, the minimum resolution of the lens should be 38 pairs of lines/mm. For a 1/3-inch format camera, the resolution of the lens should be greater than 50 pairs of lines. The smaller the target surface of the camera, The higher the resolution of the lens.

(2) Infrared sensing capability

Since the refractive index of glass is different for different wavelengths of light, the position of the focus point will be different. At present, ordinary lenses on the market can collect light with wavelengths of about 250 nm on the same plane, that is, 430-650 nm or 650~ Light in the 900nm range can be focused successfully, showing a clear image, which is why the normal lens is clear during the day, the night vision is blurred, or the night vision is clear, and the daytime is blurred.

The professional infrared lens uses a special lens (ultra-low dispersion), which can concentrate the light in the range of 430-900 nm or even longer on the same plane, so it is clear whether day or night.

Professional infrared lens is very expensive due to the special lens material. In order to reduce the price, some manufacturers use the coating method to modify the light with special materials, and can also partially achieve the effect of the infrared sensor lens, but it is not good. The coating technology is easy to fall off and evaporate, but it will affect the sharpness of the original image of the lens.

3. Correct use of the camera lens

Equipment selection must be combined with the needs of the site environment, and considering the certain economics, the following will discuss how to properly select the camera lens in terms of the use environment.

(1) Selection of lens focal length

The selection of the focal length of the camera lens is generally calculated according to the size of the imaging target surface (width w, height h), the size of the monitoring range (width W or height H), and the distance (L) of the monitoring point from the camera. Formula, calculate the focal length required for the lens.

Formula 1: f=W*L/w (W, L unit M, f, w unit mm)

Formula 2: f=H*L/h (H, L unit M, f, h unit mm)

Example: When using a 1/2′′ lens, the image size is u=4.8mm, h=6.4mm.

The distance from the lens to the scene is D=3500mm, and 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 is determined by the camera viewfinder CCD).

Substituting the above parameters into Equation 1, you can get f=4.8

3500/2500=6.72mm, so you can use a 6mm fixed focus lens. (2) Choice of lens C and CS interface

All camera lenses are threaded, and the CCD camera's lens mount interface has two industry standards, the C mount and the CS mount. The threaded portions are the same, but the distance between the two is different from the lens to the photosensitive surface.

C mount: The distance from the lens mount datum to the focus is 17.526mm.

CS mount: Special C installation, the distance from the lens mounting reference surface to the focus is 12.5mm.

When selecting the model, be sure to pay attention to the matching of the camera and lens mounting methods, that is, the lens and camera mount should be the same. The depth of the thread based on the C mount is larger than that of the CS. The lens of the CS cannot be mounted on the camera of the C mount. However, when a C mount lens can be mounted on a CS mount camera, the lens is required. Converter (filler for increasing depth).

4. Matching of lens and CCD size

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; What is the situation with a 3" CCD and a 1/2" CCD camera with the same focal length? What is the situation? The 1/3" CCD camera angle will be significantly reduced compared to the 1/2" CCD camera, while the 1/3" CCD camera The image is magnified on the monitor by an image larger than the 1/2" CCD, resulting in the effect of using a long focal length lens.

In addition, we must pay attention to the principle when selecting the lens: the CCD of the small-sized target surface can use the lens of the large-size target CCD camera, and vice versa. The reason is: If a 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, the color will become better, and the image will be The effect will definitely get better. Of course, in combination with various factors, it is better for the camera to choose a lens that matches it.

(1) Selection of manual and automatic iris lenses

The choice of manual and automatic iris lenses depends on whether the illumination of the environment is constant.

For the case where the ambient illumination is constant, such as in the elevator car, in the closed corridor, or in the room without direct sunlight, the manual iris lens can be selected, which can be adjusted once in the initial installation and commissioning according to the actual illumination of the environment. The size of the lens aperture can be obtained with a satisfactory brightness screen.

For situations where the ambient illumination is constantly changing, such as in the foyer, window, and lobby where the illumination varies greatly with the sunshine time, an auto iris lens (must be equipped with a camera with an auto iris lens socket) is required. Automatic adjustment of the brightness of the screen is achieved, and a good monitor screen with relatively constant brightness is obtained.

The control signals for the auto 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 the auto iris lens, the connection mode of the camera auto iris lens socket, and the selection of the driving mode switch of the auto iris lens.

(2) Selection of fixed focus and zoom lens

The selection of the fixed focus and zoom lens depends on the size of the scene being monitored and the clarity of the scene to be monitored.

The lens specifications (the lens specifications are generally divided into 1/3′′, 1/2′′, 2/3′′, etc.), the relationship between the focal length of the lens and the angle of view of the lens is: the longer the focal length of the lens, the field of view of the lens The smaller the angle is, the larger the lens size is, the larger the lens size is, the larger the field of view of the lens is. The above relationship shows that the lens distance is constant. In the case where the focal length of the lens becomes larger, the range of the scene of the monitored scene seen on the end monitor of the system is smaller, but the details of the screen become more and more clear; and as the lens size increases, The range of the scenes of the monitored scene seen on the end monitor of the system increases, but the details of the screen become increasingly blurred.

In a small monitored environment, such as an elevator car, small rooms should use short focal length wide-angle or super wide-angle fixed-focus lens. For example, the lens size is 1/2′′, CS type interface, lens focal length is 3.6mm or 2.6mm. For the lens, the angle of view of these lenses is not less than 99° or 127°. When the camera is generally about 2.5m in a small space, the angle of view of the lens is sufficient to cover the entire closely monitored space at close range. According to the actual situation on the spot, a manual zoom lens such as the Nissan ComputerT2Z2814CS-2 lens is used. This lens is a 1/3′′ CS type interface manual iris lens with a focal length of 2 times adjustable (manual focus adjustment). The focusing range is 2.8~6.0mm, and the angle of view varies from 96° to 47.2°. This lens is very suitable for use in a small monitored environment. It can be easily monitored according to actual needs during use. The monitoring effect of the "point" or "face" of the scene.

(3) Use of motorized zoom lens

For a general zoom (times) lens, since the minimum focal length is usually about 6.0 mm, the maximum field of view of the zoom lens is about 45°, such as using this lens for such a small surveillance. In the environment, the monitoring dead angle will inevitably increase. Although the front end pan/tilt can be operated and controlled to reduce this monitoring dead angle, this will increase the engineering cost of the system (the system needs to increase the front-end decoder, pan/tilt, protection). The cover, etc., and the complexity of the system control, so in this environment, it is not appropriate to use a zoom (times) lens.

In an open monitored environment, first of all, according to the degree of openness of the monitored environment, the user needs to see the clarity of the scene of the monitored scene on the monitor at the end of the system, and between the center point of the monitored scene and the camera lens. The linear distance is the reference basis. Under the premise that the straight line distance is sufficient and the image of the entire monitored scene is covered, the long focal length lens should be considered as much as possible, so that a monitored scene with clear details can be obtained on the system end monitor. Picture. In this environment, you can also consider the use of a zoom (times) lens (electric three-variable lens), which can be determined according to the design requirements of the system and the performance and price ratio of the system. Two points should also be considered when selecting: the shortest adjustment When the focal length (see panorama) should meet the requirements of covering the main monitored scenes;

When adjusting to the longest focal length (see details) should meet the requirements for observing the details of the scene being monitored.

Under normal circumstances, in the indoor warehouse, workshop, factory and other environments generally use 6 times or 10 times the lens to meet the requirements, and in the outdoor reservoir area, dock, square, station and other environments, can be selected according to actual requirements 10 Multiplier, 16x or 20x lens (in general, the larger the lens magnification, the higher the price, the high-magnification zoom lens or the fastball can be appropriately selected under the premise of considering the system cost.)

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