Basic knowledge of optical lens
The optical lens is an essential part of the machine vision system, which directly affects the quality of the image and the implementation and effect of the algorithm. In addition, the only way for the industry to flourish is to choose the right lens and reduce the cost of the machine vision system. Optical lens specifications range, sometimes unavoidably dizzy. Optical lens can be divided into short focus lens, medium focus lens and long focus lens from focal length. From the field of view, there are wide-angle, standard and telephoto lenses. The structure includes fixed aperture fixed focus lens, manual aperture fixed focus lens, automatic aperture fixed focus lens, manual zoom lens, automatic zoom lens, automatic aperture electric zoom lens, electric three-variable (aperture, focal length and focus can be changed) lens, etc. According to the experience we use, optical lenses in Russia are cheap.
Structural points:
1. Fixed aperture fixed focus lens
Simple. The lens has only one focus adjustment ring that can be adjusted manually. Rotate the ring left and right to make the image on the CCD target surface the clearest. Without the aperture adjustment ring, the aperture cannot be adjusted, and the luminous flux entering the lens cannot be changed by changing the lens factor, but can only be adjusted by changing the illuminance of the field of view. The structure is simple and the price is cheap.
Picture
2. Manual aperture fixed focus lens
Compared with fixed aperture fixed focus lens, manual aperture fixed focus lens has added aperture adjustment ring. The aperture range is generally from F1.2 or F1.4 to fully closed, which can easily adapt to the illuminance of the shooting scene. The aperture adjustment is manually carried out. Illumination is more uniform, the price is cheaper.
3. Auto aperture fixed focus lens
Add a gear-driven micro-motor to the aperture adjusting ring of the manual aperture fixed-focus lens, and lead out 3 or 4-core shielding wires from the driving circuit and connect them to the automatic aperture interface seat of the camera. When the light flux into the lens changes, the charge generated by the camera CCD target surface changes accordingly, so that the video signal level changes, and a control signal is generated to the automatic aperture lens, so that the motor in the lens does the corresponding Forward or reverse rotation to complete the task of adjusting the size.
4. Manual aperture zoom lens
The focal length is variable, there is a focal length adjustment ring, which can adjust the focal length of the lens within a certain range, its variable ratio is generally 2 to 3 times, and the focal length is generally 3.6 to 8mm. In practical application, the market angle of the monitored market can be easily selected by manually adjusting the zoom ring of the lens. However, when the camera installation position is fixed, it is inconvenient to adjust the zoom manually frequently. Therefore, after the completion of the project, the focal length of the manual zoom lens is generally rarely adjusted. Only play the role of fixed focus lens.
5. Automatic aperture electric zoom lens
Compared with the auto-aperture fixed-focus lens, two micro-motors are added. One of the motors is connected to the zoom ring of the lens, and the focal length of the lens can be controlled when it rotates. The focus of the lens can be completed when the other motor is connected to the focus ring of the lens. However, since two motors are added and the number of lens groups is increased, the volume of the lens increases accordingly.
6. Electric three-variable lens
Compared with the automatic iris electric zoom lens, only the control of the aperture adjustment motor is changed from automatic control to manual control by the controller.
On occasion:
According to the size of the field of view is divided into: small field of view lens, ordinary lens (about 50 degrees), wide-angle lens and ultra-wide-angle lens (100-120 degrees)
1. Standard lens: The angle of view is about 50 degrees, which is also the angle of view that a person can see without turning his head and eyes, so it is also called a standard lens. The focal length of the standard lens of 5mm camera is 40mm,50mm or 55mm. The standard lens focal length of 120 cameras is mostly 80mm or 75mm. The larger the CCD chip, the longer the focal length of the standard lens.
2. Wide-angle lens: the viewing angle is above 90 degrees, which is suitable for shooting scenes with short distance and large range, and can deliberately exaggerate the strong sense of distance and perspective in the foreground. A typical wide-angle lens for a 35mm camera has a focal length of 28mm and a viewing angle of 72 degrees. The 50,40mm lens of the 120 camera is equivalent to the 35,28mm lens of the 35mm camera.
3, long focal length lens: suitable for shooting distance of the scene, the depth of field is small easy to make the background fuzzy subject prominent, but the volume is heavy and the dynamic subject focus is not easy. 35mm camera long focal length lens is usually divided into three levels, 135mm below is called medium focal length, 135-500mm is called long focal length, 500mm 120 camera 150mm lens is equivalent to 35mm camera 105mm lens. Because the lens with long focal length is too heavy, the design of the far lens, that is, adding a negative lens behind the lens to move the main plane of the lens forward, can use a shorter lens to obtain the effect of long focal length of the lens.
4. Reflective telescopic lens: It is another design of super telescopic lens. It uses reflective mirror to form an image. However, due to the design, it is impossible to install an aperture, and only the shutter can be used to adjust the exposure.
5. Macro lens: In addition to making very close macro photography, it can also be telephoto.
The interface type is divided:
1. C- type lens
The flange focal length is the distance between the mounting flange and the convergence point of the parallel light incident on the lens. The flange focal length is 17.526mm or 0.690in. The lens can be used in linear array sensors up to 0.512in(13mm) in length. However, due to the geometric distortion and market angle characteristics, it is necessary to identify whether the short focus lens is suitable. For example, a lens with a focal length of 12.6mm should not use a linear array with a length greater than 6.5mm. If the distance from the lens to the array is determined by the flange focal length size, the lens adapter shall be added when the object magnification is less than 20 times. The loop is added behind the lens to increase the distance from the lens to the image, so that the focus range of most lenses is 5-10%. The lens extension is the focal length/object magnification.
2. CS type lens
3. U-shaped lens
A zoom lens with a flange focal length of 47.526mm or 1.7913in and a mounting rib of M42 × 1. Mainly designed for 35mm photo applications (such as domestic and imported various 135 camera lenses), for any array with a length of less than 1.25in(38.1mm). It is recommended not to use a short focal length lens. 442mm lens, 3L type lens fixed focal length wide field of view lens, originally designed for photographic magnification (such as domestic various magnifier lens), and has good characteristics in 2.25in(63.5mm) field of view. The flange focal length is a function of the specific lens. The mounting thread is M39 x 1. For arrays up to 1.25in(35.1mm) in length without limitation.
4. Special lens
Such as microscopic amplification system. Pay special attention to the difference between CS and C. When different types of camera and different types of Len are connected, the adapter ring should be customized. The main parameters of the optical lens and the evaluation of the main parameters are focal length, field of view, object distance, aperture, shutter and so on.
There is no better evaluation of the lens than MTF. However, due to aberrations (for calibration reasons), each range of the lens has an MTF value. These ranges refer:
1) Proximal part
2) Off-axis section
3) When the optical system has asymmetric distortion, the sub-parts of the above two parts in different directions. Each part has its own corresponding MTF value for different radiation energy wavelength ranges. MTF is the most commonly used and optimal index to evaluate imaging system, and it is also the optimal index to guide the integration of machine vision system.
Aperture common sense
The camera lens has a device to control the amount of light transmitted, called the aperture. If the aperture is large, the amount of light transmitted will be large. If the aperture is small, the amount of light transmitted will be small, but the aperture alone cannot fully describe the intensity of light acting on the film. The distance between the lens and the film is also related, that is, it is related to the focal length of the lens. If the focal length is small, the aperture is closer to the film, and the effect of light is stronger.
Function of Aperture
1. Control the penetration of the amount of light: the intensity of light passing through the lens to the film is inversely proportional to the square of the aperture coefficient, that is, the light intensity of the two adjacent levels of aperture acting on the film is twice the relationship.
2. Adjust the depth of field: The so-called depth of field is the range of clear imaging before and after the focus of the lens. It is related to the focal length of the lens, the aperture, and the distance of the subject. The shorter the focal length of the lens, the smaller the aperture, the greater the distance between the subjects and the greater the depth of field, the greater the clear range. And vice versa, so in terms of aperture, the small aperture has a large depth of field, which clearly and finely shows the sharp sense of distance. Large aperture with small depth of field can make the main body prominent and show the blur of the front and back themes outside the main body. It is worth mentioning that if the front and back scenes are clear, A small aperture should be used, but it should be small enough to cover the desired depth of field. It need not be too small. Too much urine will be affected by diffraction and reduce its resolution power.
Shutter common sense
The shutter of the camera is the organ that controls the exposure time. The shutter is often closed to prevent the exposure of the film installed in the camera. When shooting, it is opened and closed to allow the image light through the lens to act on the film. Early mount cameras did not have a shutter mechanism. Only one lens cover is set on the lens. When shooting and focusing, remove the lens cover, put on the lens cover and then install the photosensitive film. When shooting, lift the lens cover for a moment and then cover it. Now the shutter has been advanced to the mechanical or electronic manipulation of the opening time. The functions of the shutter are as follows:
1. The length of the opening time is used to control the amount of light penetrating. If the brightness of the subject and the aperture of the lens remain unchanged, the shutter opening time is long, the amount of light acting on the film is large, the shutter opening time is short, and the amount of light acting on the film is small. The length of time the shutter is open is called the shutter speed (Shutterspeed). In order to facilitate the adjustment of exposure, the lens of the lens shutter camera is equipped with a shutter speed adjustment ring, and the body of the focal plane shutter camera (more common) is equipped with a shutter speed adjustment ring. On the shutter speed adjustment ring, the shutter speed is divided into several stages.
2. Cooperate with aperture exposure: Only when the photography exposure is correct can photos with rich levels and clear details be produced. Exposure to be correct, must be in accordance with the film speed and aperture strength, the shutter speed and aperture with the appropriate. To control the amount of light entering the lens, it is necessary to control the so-called "aperture stop" of the lens. Aperture diaphragm is located inside the lens, usually by a number of movable metal blades (called diaphragm blades), can make the middle of the formation of (approximate) circular hole larger or smaller, in order to achieve the purpose of controlling the amount of light through the size.