Lens parameter index

Optical lens is generally called camera lens or photographic lens, and its function is optical imaging. In the machine vision system, the main function of the lens is to focus the imaging target on the photosensitive surface of the image sensor. The quality of lens directly affects the overall performance of machine vision system; The reasonable selection and installation of optical lens is an important link in the design of machine vision system.

1. lens related parameters

(1) focal length

Focal length is an important parameter of optical lens, which is usually expressed by F. The size of focal length determines the size of field of view. Small focal length, large field of view and large observation range, but the resolution of distant objects is not clear. Large focal length, small field of view and small observation range. As long as the focal length is chosen properly, even distant objects can be seen clearly. Because the focal length and the field of view are in one-to-one correspondence, a certain focal length means a certain field of view, so when choosing the focal length of the lens, we should fully consider whether the observation details are important or the observation range is large. If we want to see the details, we should choose a long focal length lens. For close-up scenes, choose a wide-angle lens with a small focal length.

(2) aperture coefficient

That is, luminous flux, expressed by f, is measured by the ratio of lens focal length f to aperture d, and each lens is marked with the maximum f value, for example, 6mm/F 1.4 means that the maximum aperture is 4.29mm The luminous flux is inversely proportional to the square of the f value, and the smaller the f value, the greater the luminous flux. The scale values of the aperture index sequence on the lens are 1.4, 2, 2.8, 4, 5.6, 8, 1 1, 16, 22, etc. The rule is that the exposure of the previous scale value is exactly twice that of the latter scale value. That is to say, the aperture of the lens is11.4, 1/2, 1/2.8, 1/4, 1/5.6, 1/8 respectively.

(3) Depth of field

Focusing on a scene during photography will form a clear area before and after the scene. This blank area is called Quan Jingshen, or depth of field for short. Three basic factors that determine the depth of field:

Aperture Aperture is inversely proportional to depth of field. The larger the aperture, the smaller the depth of field.

The focal length is inversely proportional to the depth of field. The greater the focal length, the smaller the depth of field.

The object distance is proportional to the depth of field. The greater the object distance, the greater the depth of field.

(4) Spectral characteristics

The spectral characteristics of optical lens mainly refer to the transmission characteristics of optical lens to light in various bands. In some machine vision application systems, it is required that the color of the image is highly consistent with the color of the imaging target. Therefore, it is hoped that the spectral composition of each band will not change when it passes through the optical lens, except for a certain loss of total intensity.

The main factors affecting the spectral characteristics of optical lenses are the interference characteristics of thin films and the absorption characteristics of glass materials. In the machine vision system, in order to make full use of the resolution of the lens, the spectral characteristics of the lens should match the use conditions. That is to say, the light with the highest resolution of the lens should match the illumination wavelength and acceptance wavelength of the CCD device, and the light transmittance of the optical lens at this wavelength should be improved as much as possible.

(5) Resolution of the lens

The intrinsic index to describe the imaging quality of the lens is the optical transfer function and distortion of the lens, but for users, all they need to know is the spatial resolution of the lens. Taking the number of black and white stripes per millimeter as the unit of measurement, the calculation formula is: lens resolution n = 180/ frame height. Because the CCD target size of the camera has been standardized, such as 1/2 inch camera, its target size is 6.4mm* width * 4.8mm high, and 1/3 inch camera is 4.8mm wide * 3.6mm high. Therefore, for 1/2 inch CCD target, the minimum resolution of the lens should be 38 lines /mm, and for 1/3 inch camera, the resolution of the lens should be greater than 50 lines. The smaller the target of the camera, the higher the resolution of the lens.

(6) Aperture or luminous flux

The luminous flux of a lens is measured by the ratio of the focal length of the lens to the luminous flux aperture. Marked by F, each lens is marked with its maximum F value, and the luminous flux is inversely proportional to the square of F value. The smaller the f value, the larger the aperture. Therefore, according to the degree of light change in the monitored part, choose to use manual aperture or automatic aperture lens.

(7) lens interface

There are many different types of interfaces between lens and camera. Common industrial cameras include C interface, CS interface, F interface, V interface, T2 interface, Leica interface, M42 interface, M50 interface and so on. The different interface types are not directly related to the lens performance and quality, but the different interface modes. Generally, you can also find the interfaces between various common interfaces.

According to the classification of lens installation, all camera lenses are threaded. There are two industry standards for lens installation of CCD cameras, namely C-mount and CS-mount. Both of them have a thread length of 1 inch, but the difference between them lies in the distance between the lens and the sensor after the lens is installed in the camera:

CS-mount: The distance from the image sensor to the lens should be12.5mm..

C-mount: The distance between the image sensor and the lens should be17.5mm. A 5mm washer (C/CS connecting ring) can be used to convert the C-type mounting lens into the CS-type mounting lens.

2. Interaction between lens parameters.

(1) Influence of focal length:

The smaller the focal length, the greater the depth of field;

The smaller the focal length, the greater the distortion;

The smaller the focal length, the more serious the vignetting phenomenon, which reduces the illumination of the aberration edge;

(2) The influence of aperture size:

The larger the aperture, the higher the brightness of the image;

The larger the aperture, the smaller the depth of field;

The larger the aperture, the higher the resolution;

(3) the center and edge of the image field

Generally, the resolution of the image field center is higher than that of the edge;

Generally, the central illumination of the image field is higher than that of the edge light field;

(4) Influence of light wave wavelength:

Under the same camera and lens parameters, the shorter the wavelength of the light wave of the illumination source, the higher the resolution of the obtained image. Therefore, it is very important to use monochromatic light with short wavelength as illumination source to improve the accuracy of the vision system which needs accurate size and position measurement.

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