Introduction to the classification of camera lenses

The classification of the lens can be roughly divided into: (1) Wide-angle lens: The angle of view is above 90 degrees, and is generally used in places with small visual distances and large visual angles such as elevator cars, halls, etc.; (2) Standard lens: the angle of view is about 30 degrees, and is generally used in places such as walkways and community perimeters; (3) Telephoto lens: the angle of view is within 20 degrees, the focal length ranges from tens of millimeters to hundreds of millimeters, used for long-distance surveillance (4)Zoom lens: The focal length of the lens can be changed from wide-angle to telephoto, which is used in areas with large depth of field and wide range of angle of view. (5) Pinhole lens: used for concealed monitoring. When determining the focal length of the lens, there are the following five factors to determine the lens standard: (1) Monitor the size of the site; (2) The size of the subject; (3) Object distance; (4) Focal length; (5) CCD target surface size. The first 4 points can be measured on site and calculated to determine the focal length standard of the lens. The calculation method is as follows: u 1/3″CCDF=4.8×L/W or F=3.6×L/H u 1/2″CCDF=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 1/3” and 1/2” CCD cameras, the focal length of the 1/3” CCD camera lens must be shortened; on the contrary, if the same focal length is used in 1/3” CCD and 1/2” CCD cameras What about the lens? The viewing angle of 1/3″ CCD camera will be significantly smaller than that of 1/2″ CCD camera, and the image of 1/3″ CCD camera will be enlarged on the monitor than that of 1/2″ CCD. , Produced the effect of using a long focal length lens. In addition, we should pay attention to this principle when choosing a lens: that is, a small-size target CCD can use a large-size target CCD camera lens, and vice versa. The reason is: if a 1/2" CCD camera adopts a 1/3" lens, the amount of light entering will become smaller, the color will become worse, and even the image will be missing; otherwise, the amount of light entering will become larger, the color will become better, and the image The effect will definitely get better. Of course, considering various factors, the camera is best to choose a lens that matches it. Selection of manual iris and automatic iris The lens aperture is divided into manual and automatic. In the past, due to the use of cameras outdoors or other special occasions, automatic iris lenses were often used. In the current monitoring projects, due to the extensive use of CCTV systems in smart buildings, indoor monitoring points account for a relatively high proportion. Many engineering companies also like to use automatic iris lenses when making engineering equipment quotations. Although the automatic iris lens is more adaptable to the light changes of the monitoring point, its price is significantly higher than that of the manual fixed focus lens of the same focal length. However, most cameras now have electronic shutters, 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 currently used in the market are divided into two Major categories: a. Power-driven automatic iris lens; b. Video-driven automatic iris lens. The power-driven automatic iris lens controls the lens through four wires, two of which are DC12V or DC24V power to drive the motor in the lens, and the other two control wires sense the illuminance of the external light source through the light sensing point in the lens to control the iris Size: The video-driven automatic iris lens controls the lens through three wires, one of which is a video trigger signal to start the iris and control the size of the iris, and the other two are DC12V or DC24V power lines to drive the motor motor. Although most black-and-white or color cameras on the market have an automatic iris lens interface, except for a few that are compatible with the two lenses, most cameras are not compatible and can only use power-driven automatic iris lenses or video-driven automatic iris lenses. If some cameras are damaged during use, the newly purchased camera will have 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 varies greatly, and the ambient illuminance in the summer sun can reach 50000Lx-100000Lx; when the street light at night is only 10Lx, The magnitude of the change is considerable. 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 it is impossible to achieve the effect of controlling the image. In the TV monitoring system, it is very important to choose the correct camera lens according to the monitoring environment of the scene, because it directly affects whether the effect of the monitored surface picture seen on the end monitor of the system after the system is composed can satisfy the system's performance. Design requirements (in terms of screen range or image details), so the correct selection of camera lens can optimize the system design and obtain a good monitoring effect. To The camera lens can be divided into two kinds of manual iris lens and automatic iris lens in terms of aperture, and can be divided into two kinds of fixed focus lens and zoom lens in terms of focal length. Now let's talk about how to choose the camera lens correctly according to the different use environment. 1. Selection of manual and automatic iris lens The selection of manual and automatic iris lens depends on whether the illuminance of the environment is constant. In the case of constant environmental illuminance, such as in the elevator car, enclosed corridor, and room without direct sunlight, manual iris lens can be selected, so that it can be adjusted at one time according to the actual illuminance of the environment during the initial installation and debugging of the system The aperture size of the lens is sufficient to obtain a satisfactory brightness picture. For situations where the ambient illuminance is constantly changing, such as halls, windows, and lobbies where the illuminance varies greatly with the time of daylight, an automatic iris lens (must be equipped with a camera with an automatic iris lens socket) is required, so that you can Realize the automatic adjustment of the screen brightness, and obtain a good and relatively constant brightness monitoring screen. The control signal of 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. Pay attention to the coordination of the three. To 2. Selection of fixed focus and zoom lenses The selection of fixed focus and zoom lenses depends on the size of the monitored scene range and the required clarity of the scene to be monitored. Lens specifications (lens specifications are generally divided into 1/3", 1/2" and 2/3", etc.). Under certain circumstances, 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 (see Figure 1); when the focal length of the lens is fixed, the relationship between the lens specification and the lens field of view angle is: the larger the lens specification, the larger the field of view angle of the lens. Therefore, the above relationship It can be seen that when the lens object distance is fixed, as the focal length of the lens becomes larger, the screen range of the monitored scene seen on the monitor at the end of the system becomes smaller, but the screen details become clearer and clearer; With the increase of lens specifications, the screen range of the monitored scene seen on the monitor at the end of the system will increase, but the details of the screen will become more and more blurred. Under the premise of certain lens specifications and lens focal length, CS-type interface lenses The angle of view will be greater than that of the C-mount lens. The field 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 greater than the vertical field of view of the image. Generally speaking, the field of view generally refers to the horizontal field of view of the image of the lens. . In a small monitored environment, such as an elevator car, a short focal length wide-angle or ultra-wide-angle fixed-focus lens should be used in small rooms. If the lens size is 1/2", the CS type interface is used, and the lens focal length is 3.6mm or 2.6mm Lens, the field of view angle of these lenses is not less than 99° or 127°. For the camera in a small space, when the elevation is about 2.5m, the field of view of the lens is enough to cover the entire close-up narrow space to be monitored. According to the actual situation of the scene, a manual zoom lens such as Nissan Computar T2Z2814CS-2 lens is selected. This lens is a 1/3" CS type interface manual iris lens, and its focal length is adjustable 2 times (manual focus). The focusing range is 2.8～6.0mm, and the field angle change range is 96°～47.2°. This kind of lens is very suitable for use in a small monitored environment. It can be used conveniently and flexibly according to actual needs. The monitoring effect of the "point" or "surface" of the scene. For a general zoom (x) lens, since its minimum focal length is usually about 6.0mm, the maximum field of view of its zoom (x) lens is about 45°. If this lens is used for such a narrow surveillance In the environment, the monitoring blind angle will inevitably increase. Although the front-end pan/tilt can be operated and controlled to reduce such monitoring blind angles, this will inevitably increase the engineering cost of the system (the system needs to increase the front-end decoder, pan/tilt, protection Cover, etc.), as well as the complexity of the system control, so in this environment, zoom (times) lens should not be used. 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 between the center point of the monitored scene and the camera lens The straight-line distance is a reference basis. Under the premise that the straight-line distance is fixed and the entire monitored scene is covered, a long focal length lens should be considered as much as possible, so that a clearer detailed monitored scene can be obtained on the end monitor of the system. Picture. In this environment, zoom (times) lens (electric three-variable lens) can also be considered. This 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 (viewing the panorama), it should be able to meet the requirements of covering the main monitored scenes; (2) When adjusting to the longest focal length (see details), it should be able to meet the requirements for observing the details of the scene being monitored. Under normal circumstances, in indoor warehouses, workshops, factories and other environments, 6x or 10x lenses can generally be selected to meet the requirements, while in outdoor warehouse areas, docks, squares, stations and other environments, 10x lenses can be selected according to actual requirements. Magnification, 16x, or 20x lens is fine (generally, the larger the lens magnification, the higher the price, and a high-magnification zoom lens can be appropriately selected under the premise of comprehensive consideration of the system cost).