Optical lens is generally referred to as camera lens or photographic lens, referred to as the lens, its function is optical imaging. The lens is an important component of the machine vision system, which plays a key role in imaging quality. It has an impact on several main indicators of imaging quality, including: resolution, contrast, depth of field and various aberrations. There are not only a wide variety of lenses, but also a very large difference in quality, but the general user often does not pay enough attention to the choice of lens when designing the system, which leads to the failure of the ideal image and even the failure of the system development. The purpose of this paper is to summarize the relationship between various factors by introducing the classification and main parameters of various common lenses, so that readers can master the skills of lens selection in machine vision system.
Division according to the size of the effective image field
The camera lens is installed at the front end of a large telescopic camera obscura, and a large frosted glass is installed at the rear end of the camera obscura. When the lens aperture is opened to the maximum and focused on the infinite vision object, the images presented on the frosted glass are located in a circular area, while the outside of the circle is dark and there is no image. This circular area with an image is called the maximum image field of the lens. In the center of this maximum image field, there is an area that can form a clear image of the scenery at infinity, which is called a clear image field. The target surface of the camera or camera is generally located within the clear image field, and this limited range is called the effective image field. Since the target surface size of the camera used in the vision system has various models, when selecting the lens, it must be noted that the effective image field of the lens should be greater than or equal to the target surface size of the camera, otherwise the corner part of the imaging will be blurred or even no image.
According to the size of the effective image field, it can be generally divided into the following categories:
Classification according to focal length
According to the focal length can be adjusted, can be divided into fixed focal length lens and zoom lens two categories. According to the length of the focal length, fixed focal length lens can be divided into fisheye lens, short focus lens, standard lens, telephoto lens four categories. It should be noted that the length of the focal length is not divided by the absolute value of the focal length as the primary standard, but by the size of the image angle as the main basis for distinction, so when the size of the target surface is not equal, the focal length of the standard lens is also different. Zoom lens has a zoom ring, adjust the ring can make the focal length of the lens within a predetermined range of flexible change. The ratio of the longest focal length value and the shortest focal length value of the zoom lens is called the zoom magnification of the lens. Zoom lens can be divided into manual zoom and electric zoom two categories.
The zoom lens is widely used in the field of photography because it can continuously change the focal length value and is very convenient to use when the photographic field of view needs to be changed frequently. However, due to the large number of lenses and complex structure of the zoom lens, the maximum relative aperture cannot be made too large, resulting in low image brightness and poor image quality. At the same time, it is difficult to correct the aberration for various focal lengths and focusing distances in the design, so its imaging quality cannot be compared with the fixed focal length lens of the same grade.
The focal length of the lens commonly used in practice is from 4mm to 300mm, there are many grades, how to choose the appropriate focal length of the lens is a major problem to be considered in the design of the machine vision system. The imaging law of optical lens can be derived according to two basic imaging formulas Newton formula and Gauss formula, for the common design model of machine vision system, we generally choose the appropriate focal length lens according to the two conditions of imaging magnification and object distance, here gives a set of practical calculation formula:
Divided according to lens interface type
There are many different types of interfaces between the lens and the camera. Industrial cameras commonly used include C interface, CS interface, F interface, V interface, T2 interface, Leica interface, M42 interface, M50 interface, etc. The different types of interfaces are not directly related to the performance and quality of the lens, but the different interface methods. Generally, the conversion interface between various commonly used interfaces can also be found.
The C interface and the CS interface are the most common international standard interfaces for industrial cameras. They are 1 inch-32UN inch threaded connection. The threaded connection of the C interface and the CS interface is the same. The difference is that the rear intercept of the C interface is 17.5mm, and the rear intercept of the CS interface is 12.5mm. Therefore, the camera with CS interface can be connected with the lens of C port and CS port, but a 5mm adapter ring is required when using the lens of C port. The camera with C interface cannot use the lens of CS port.
F interface lens is the interface standard of Nikon lens, so it is also called Nikon mouth. It is also a common type in industrial cameras. Generally, when the target surface of the camera is larger than 1 inch, the lens with F mouth is required.
V interface lens is the main standard used by the famous professional lens brand Schneider lens, and is generally used for the camera target surface or special purpose lens.
special purpose lens
Microscope lens (Micro), generally refers to the imaging ratio is greater than 10:1 shooting system used, but because the current camera pixel size has been achieved within 3 microns, so the general imaging ratio is greater than 2:1 will also choose microscope lens.
Macro lens (Macro), generally refers to the imaging ratio of 2:1~1:4 in the range of special design of the lens. In the case of image quality requirements are not very high, the general can be used between the lens and the camera to add close-up ring or in front of the lens to achieve the effect of magnifying imaging.
Telecentric lens (Telecentric), mainly to correct the parallax of the traditional lens and special design of the lens, it can be in a certain range of object distance, so that the image magnification will not change with the change of object distance, which is measured on the object is not on the same surface of the situation is a very important application.
Application example of telecentric lens
Ultraviolet lens (Ultraviolet) and infrared lens (Infrared), the general lens is designed for the use of visible light range, due to the different refractive index of the same optical system for different wavelengths of light, resulting in the same point of different wavelengths of light imaging can not be concentrated into a point, resulting in chromatic aberration. The achromatic design of commonly used lenses is also aimed at the visible range. Ultraviolet lenses and infrared lenses are specially designed for ultraviolet and infrared lenses.