How to choose the right camera lens

1. Selection of manual and automatic aperture lens The selection of manual and automatic aperture lens depends on whether the illumination of the use environment is constant. For situations where the ambient illumination is constant, such as in elevator cars, closed corridors, and rooms without direct sunlight, manual aperture lenses can be selected, which can be set at one time according to the actual illumination of the environment during the initial installation and commissioning of the system. Lens aperture size, you can get a satisfactory brightness picture. For situations where the ambient illuminance is constantly changing, such as in halls, windows, and halls where the illuminance changes greatly with the time of daylight, an automatic iris lens (must be equipped with a camera with an automatic iris lens socket), so that Realize the automatic adjustment of the screen brightness, and obtain a good monitoring screen with relatively constant brightness. The control signals for the auto iris lens can be divided into DC and VIDEO control, that is, DC voltage control and video signal control.

This is in the selection of the type of auto iris lens, the connection method of the auto iris lens socket of the camera, and the drive mode switch of the auto iris lens, which can be coordinated and coordinated.

2. Selection of fixed focus and zoom lens The selection of fixed focus and zoom lens depends on the size of the monitored scene range and the clarity of the required monitored scene picture. Lens specifications (lens specifications are generally divided into 1/3 ″, 1/2 ″, 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 more the field of view of the lens The smaller the angle (see Figure 1); under a given focal length of the lens, the relationship between the lens specification and the field angle of the lens is: the larger the lens specification, the larger the field angle of the lens.

Therefore, from the above relationship, it can be known that, with a fixed lens object distance, as the focal length of the lens becomes larger, the scope of the monitored scene seen on the system end monitor becomes smaller, but the details of the picture become clearer and clearer. ; With the large lens specifications, the scope of the monitored scene seen on the system's end monitor increases, but the picture details become increasingly blurred.

Under the premise of certain lens specifications and lens focal lengths, the field of view angle of the CS-type lens will be larger than that of the C-type lens. The field of view angle of the lens can be divided into the horizontal field of view angle of the image and the vertical field of view angle of the image, and the horizontal field of view angle of the image is greater than the vertical field of view angle of the image. Generally, the field of view we refer to is the horizontal field of view of the lens. . In a small monitored environment, such as an elevator car, a small room should use a short focal length wide-angle or ultra-wide-angle fixed-focus lens. If a 1/2 ”lens type is selected, the CS type interface is used, and the lens focal length is 3.6mm or 2.6mm. The angle of field of view of these lenses is not less than 99 ° or 127 °. When the general elevation of the camera in a narrow space is about 2.5m, the field of view angle of the lens is sufficient to cover the entire narrow space to be monitored. According to the actual situation of the scene, a manual zoom lens such as a Nissan Computar T2Z2814CS-2 lens is selected. 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 lens is very suitable for use in a small monitored environment. When in use, it can be conveniently implemented according to actual needs and flexibly implement monitoring of the monitored The effect of monitoring the "points" or "surfaces" of a scene. For a normal zoom (double) lens, since its minimum focal length is usually about 6.0mm, the maximum field of view of its zoom (double) lens is about 45 °. If this lens is used for such a narrow monitored In the environment, the monitoring dead angle must increase. Although the front-end PTZ can be controlled by operation control, this will increase the engineering cost of the system (the system needs to increase the front-end decoder, PTZ, and protection). Hood, etc.), and the complexity of system control, so in this environment, it is not appropriate to use a zoom (double) lens. In an open monitored environment, first of all, according to the openness of the monitored environment, the user needs to see the clarity of the monitored scene picture on the system end monitor, and the center point of the monitored scene to the camera lens. The straight line distance is used as a reference. Under the premise that the straight line distance is fixed and covers the entire scene of the monitored scene, you should consider using a long focal length lens as much as possible, so that you can obtain a monitored scene with clearer details on the system end monitor. Screen. In this environment, you can also consider using a zoom (double) lens (electric three-variable lens), which can be determined according to the system design requirements and the system's performance-price ratio. Two points should also be considered when selecting: (1) in When adjusted to the shortest focal length (looking at the panorama) should be able to meet the requirements of covering the main monitored scene picture; (2) When adjusted to the longest focal length (seeing the details) should be able to meet the requirements of observing the picture details of the monitored scene. Under normal circumstances, 6x or 10x lenses can be used to meet the requirements in indoor warehouses, workshops, factories and other environments. In outdoor warehouses, docks, squares, stations and other environments, 10 can be selected according to actual requirements. Magnification, 16x, or 20x lens is sufficient (generally, the larger the lens magnification, the higher the price, a high-magnification zoom lens can be appropriately selected on the premise of comprehensive consideration of the system cost allowance).

3. The theory of correct selection of the focal length of the lens is a very important parameter to calculate the field of view of the lens that captures the scene. The field of view of the lens changes with the focal length of the lens and the size of the camera (the relationship between the changes is as described above), covering the focal length of the scene lens It can be calculated using the following formula:

(1) f = u · D / U (2) f = h · D / Hf: 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 : Example of image width: When a 1/2 ″ lens is used, 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 determined from the following The formula calculates H = 1.333 · U, and this relationship is determined by the camera viewfinder CCD lens.) When the above parameters are substituted into formula (1), f = 4.8 · 3500/2500 = 6.72mm can be obtained, so a 6mm fixed focus lens is used. can.