Reasonable selection of camera lens is classified according to structure.
There are many kinds of lenses, which can be divided into fixed aperture fixed focus lens, manual aperture fixed focus lens, automatic aperture fixed focus lens, manual zoom lens, automatic aperture electric zoom lens and so on. Let's make a simple analysis one by one:
Fixed aperture fixed focus lens: simple. The lens has only one focus adjustment ring that can be manually adjusted, and rotating the adjustment ring left and right can make the image imaged on the CCD target surface clearest. Without the aperture adjusting ring, the aperture cannot be adjusted, and the luminous flux entering the lens cannot be changed by changing the lens factor, only by changing the illumination of the field of view. Simple structure and low price.
Manual aperture fixed-focus lens: Manual aperture fixed-focus lens has one more aperture adjusting ring than fixed-focus lens, and the aperture range is generally from F 1.2 or F 1.4 to full closure, which can easily adapt to the illumination of the shooting scene. The aperture adjustment is done manually. The illumination is relatively uniform and the price is relatively cheap.
Automatic aperture fixed-focus lens: Add a gear-driven micromotor to the aperture adjusting ring of manual aperture fixed-focus lens, and lead out 3 or 4 core shielded wires from the driving circuit and connect them to the camera automatic aperture interface seat. When the luminous flux entering the lens changes, the charge generated by the CCD target surface of the camera also changes, so that the video signal level changes, and a control signal is generated and transmitted to the automatic aperture lens, so that the motor in the lens rotates forward or backward accordingly, thus completing the task of resizing.
Manual aperture zoom lens: the focal length is variable, and there is a focal length adjustment ring, which can adjust the focal length of the lens within a certain range. The variable magnification is generally 2~3 times and the focal length is generally 3.6 ~ 8 mm In practical application, the market angle of the monitored market can be easily selected by manually adjusting the zoom ring of the lens. However, it is inconvenient to manually adjust the zoom frequently when the camera installation position is fixed. Therefore, after the completion of the project, the focal length of the manual zoom lens is rarely adjusted. It only acts as a fixed-focus lens.
Automatic aperture electric zoom lens: Compared with automatic aperture fixed focus lens, two micro motors are added, one of which is connected with the zoom ring of the lens to control the focal length when the lens rotates; The other motor is connected to the focus ring of the lens, and when it is controlled to rotate, the focus of the lens can be completed. However, due to the addition of two motors and the increase in the number of lens groups, the volume of the lens has also increased accordingly. Compared with the automatic aperture electric zoom lens, the electric three-variable lens only changes the control of the aperture adjusting motor from automatic control to manual control by the controller.
In this paper, we will mainly analyze how to choose the correct camera lens according to different use environments.
Selection of Manual and Automatic Aperture Lens
It should be said that the choice of manual and automatic aperture lens mainly depends on whether the illumination in the use environment is constant. In the case of constant ambient illumination, such as in elevator cars, closed corridors and rooms without direct sunlight, manual aperture lens can be selected, so that the aperture size of the lens can be adjusted at one time according to the actual illumination of the environment during the initial installation and debugging of the system to obtain a satisfactory brightness picture. However, if the ambient illumination is constantly changing, such as foyer, window, lobby, etc., where the illumination changes greatly with the sunshine time, it is necessary to choose an automatic aperture lens (a camera with an automatic aperture lens socket must be equipped), so that the brightness of the picture can be automatically adjusted and a good monitoring picture with constant brightness can be obtained.
The control signal of automatic aperture lens can be divided into DC and video control, namely DC voltage control and video signal control. In the selection of auto-stop lens type, the connection mode of camera auto-stop lens socket and the selection of auto-stop lens drive mode switch, the three should be coordinated well.
Selection of fixed focus and zoom lens
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 picture.
Lens specifications (lens specifications are generally divided into 1/3? 、 1/2? What about two thirds? Under certain conditions, the relationship between lens focal length and lens field of view angle is as follows: the longer the lens focal length, the smaller the lens field of view angle (as shown in figure1); When the focal length of the lens is fixed, the relationship between the lens specification and the lens field of view is that the larger the lens specification, the larger the lens field of view. Therefore, it can be seen from the above relationship: under the condition that the lens object distance is constant, with the increase of the lens focal length, the picture range of the monitoring scene seen on the monitor at the end of the system is smaller, but the picture details are clearer and clearer; With the increase of lens specifications, the picture range of the monitoring scene seen on the monitor at the end of the system increases, but its picture details become more and more blurred. On the premise of certain lens specifications and lens focal length, the field of view angle of CS interface lens will be larger than that of C interface lens.
The field of view of the lens can be divided into horizontal field of view and 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. Usually, the angle of view we say generally refers to the horizontal angle of view of a lens.
In a narrow monitoring environment, such as an elevator car, a short-focus wide-angle or ultra-wide-angle fixed-focus lens should be used in a narrow room. For example, the lens specification is 1/2? , CS interface, lens focal length is 3.6mm or 2.6mm, and the field of view of these lenses is not less than 99? Or 127? For a camera with a general elevation of about 2.5m in a narrow space, the field of view of its lens is enough to cover the whole narrow monitoring space. You can also choose a manual zoom lens such as Nissan Computeart2Z28 14Cs-2 lens according to the actual situation. This lens is 1/3? Manual aperture lens with CS interface, whose focal length is 2 times adjustable (manual focusing). The focusing range is 2.8~6.0mm and the viewing angle is 96? ~47.2? This kind of lens is very suitable for use in a narrow monitoring environment and can be used conveniently and flexibly according to actual needs. Point? Or? Face? The monitoring effect of.
For a general zoom lens, the minimum focal length is usually around 6.0mm, so the maximum field of view of the zoom lens is 45? Left and right, if this lens is used in this narrow monitoring environment, its monitoring dead angle will inevitably increase. Although this monitoring dead angle can be reduced by operating and controlling the front-end pan/tilt, it will definitely increase the engineering cost of the system (the system needs to increase the front-end decoder, pan/tilt and protective cover, etc.). ) and the complexity of system control, so it is not appropriate to use zoom (magnifying) lens in this environment.
In an open monitoring environment, firstly, according to the openness of the monitoring environment, the clarity of the monitoring scene picture required by the user on the monitor at the system end, and the linear distance between the center point of the monitoring scene and the camera lens, the telephoto lens should be considered as much as possible on the premise that the linear distance is certain and covers the whole monitoring scene picture, so as to obtain the monitoring scene picture with clearer details on the monitor at the system end. In this environment, zoom (magnifying) lens (electric three-variable lens) can also be considered, which can be determined according to the design requirements of the system and the cost performance of the system. Two points should be considered when selecting: (1) When adjusting to the shortest focal length (panorama), the requirements of covering the main monitoring scenes should be met; (2) When adjusting to the longest focal length (see), it should be able to meet the requirements of observing and monitoring the details of the scene. Usually in indoor warehouses, workshops, factories and other environments, 6 times or 10 times lens can meet the requirements, while in outdoor environments such as reservoir areas, docks, squares and stations. , 10 times, 16 times or 20 times lens can be selected according to actual requirements (generally speaking, the larger the lens multiple, the higher the price, which can be used for.
Case study: case study
In actual engineering operation, we often encounter this problem: some cameras look too far and can't see the nearby ones; Some cameras look too big to see clearly, so how to choose what zoom lens to use? Select the zoom scale to use? What is the appropriate focal length?
According to the knowledge of lens transmission principle: F/D = H/H.
Where: f- lens focal length unit: mmd- the distance between the lens and the object being photographed unit: mh- lens height unit: mmh- the actual height unit of the picture taken by the camera at the monitoring site: m Let's take an example: take a camera with 1/4CDW * H = 3.6 * 2.7mm as an example, with optical zoom 18 times.
1, assuming that Party A wants to see a person's big head clearly, what is the distance?
First of all, we need to clarify a concept that is easier for people who have taken photos with a camera to understand: if you want to see the big head, the face of the person accounts for about 1/2 of the picture, and the face is about 0.25m, then the height of the monitoring site is d = 0.25 * 2 = 0.5m ..
Substitution formula: d1= 4.1* 0.5/2.7 = 0.76m.
D2 = 73.8 * 0.5/2.7 =13.67m.
So this camera wants to see the head of the NPC clearly, so 0.76m? d？ 13.67 meters.
2. If Party A's conditions are not so high, for example, it only needs to be able to see the whole human body, and the human body should occupy 1/2 of the picture according to the average height of 1.65, then the height of the monitoring site is d = 2 *1.65 = 3.3m..
Substitution formula: d1= 4.1* 3.3/2.7 = 5.01m.
D2 = 73.8 * 3.3/2.7 = 90.2
5.0 1m at this time? d？ 90.2 meters
3. Assuming that the camera is not selected, but Party A says that it is necessary to take a clear picture of the human body beyond 150m, we can also calculate whether to use 1/4ccd:
F =150 * 2.7/3.3 = 122.73mm, that is, to achieve this effect, the longest focal length of the camera should reach122.73 mm, and if the shortest focal length is 4mm, it is 3 1 multiple zoom.