Optical lens is an indispensable component in the machine vision system, which directly affects the quality of imaging, affecting the implementation and effect of the algorithm. Optical lens from the focal length can be divided into short-focus lens, medium-focus lens, telephoto lens; from the size of the field of view has a wide-angle, standard, telephoto lens; structure is divided into a fixed aperture fixed-focus lens, manual aperture fixed lens, Automatic aperture fixed-focus lens, manual zoom lens, automatic zoom lens, auto iris motorized zoom lens, electric three variable (aperture, focal length, variable focus) lens.
Introduction
For the camera, the quality of the lens has always been a key factor affecting the quality of the image, and digital cameras are no exception.
Although the digital camera has a limited CCD resolution, in principle, the optical resolution of the lens is less demanding; on the other hand, the digital camera has a smaller imaging area (because the digital camera is imaged on the CCD, and the area of the CCD is relatively small. The traditional 35mm camera has a much smaller film, so it requires a lens to ensure a certain image quality. For example, for a certain subject, the horizontal direction needs 200 pixels to reproduce its details perfectly. If the imaging width is 10mm, a lens with an optical resolution of 20 lines/mm can be competent if the imaging width is 1mm requires the optical resolution of the lens to be over 200 lines/mm. On the other hand, traditional film is more sensitive to ultraviolet rays, and external cameras often need to install UV mirrors, while CCD is more sensitive to infrared rays. Adding special coatings or additional filters to the lens will greatly improve the image quality. The physical caliber of the lens must also be considered. Regardless of its relative caliber, the larger the physical caliber, the greater the luminous flux. The digital camera will receive and control the light better, and the image quality will be better.
For commercial or home digital cameras, some manufacturers use relatively good lenses. The Fujifilm camera uses a professional Fujinon lens with 170 lines/mm resolution. This built-in new Fujinon lens is clearer than most SLR lenses. Not only does it guarantee the quality of image capture in terms of accuracy, but it also achieves an amazing 0.3% lens error rate, which is 2/3 lower than that of a general digital camera.
In addition, in some digital cameras, there are also two types of lens, remote and wide angle. This is also a reference indicator when you choose a digital camera.
In a conventional digital camera, a wide-angle lens is a photographic lens that has a shorter focal length than a standard lens, a larger angle of view than a standard lens, a lens that is longer than a fisheye lens, and a smaller angle of view than a fisheye lens. Wide-angle lenses are divided into ordinary wide-angle lenses and ultra-wide-angle lenses. 135 camera general focal length lens is generally 38-24 mm, viewing angle is 60-84 degrees; ultra-wide-angle lens focal length is 20-13 mm, viewing angle is 94-118 degrees. Since the wide-angle lens has a short focal length and a large viewing angle, it can capture a large area of the scene within a short shooting distance. Therefore, it is widely used for shooting large-scale wind photography works. In photography, using a wide-angle lens, can get the following effects: First, to increase the spatial depth of the photographic picture; Second, a longer depth of field, to ensure that the subject before and after the scene can be clear on the screen Reproduction. Therefore, the vast majority of modern compact automatic cameras (commonly known as point-and-shoot cameras) use 38-35 mm ordinary wide-angle lens; third, the lens covers a large area, shooting a wide range of scenery; Fourth, the scene taken at the same shooting distance Compared with the scene shot using the standard lens, the image on the screen is small. Fifth, the distortion of the perspective and the distortion of the image are prone to appear in the screen. The shorter the focal length of the lens is, the closer the shooting distance is, the more obvious this kind of defect is.
Commercial-grade digital cameras often use the same ordinary wide-angle lens as a normal 35-mm camera. Due to its deep depth of field and wide shooting range, the digital camera with the same performance can have wide-angle and long-distance when selecting a digital camera. Digital cameras will perform better.
classification
By structure
Fixed aperture prime lens
Simple: The lens has only one focus adjustment ring that can be manually adjusted. Rotating the ring left or right allows the image on the CCD target surface to be sharpest. Without the iris adjustment ring, the aperture cannot be adjusted. The flux entering the lens cannot be changed by changing the lens factor. Can only be adjusted by changing the illuminance of the field of view. The structure is simple and the price is cheap.
Manual aperture prime lens
The manual aperture fixed lens has an aperture adjustment ring that is larger than the fixed aperture fixed lens. The aperture range is generally from F1.2 or F1.4 to fully closed. It can easily adapt to the illumination of the scene to be shot. The aperture adjustment is performed manually. of. The illumination is more uniform and the price is cheaper.
Automatic aperture prime lens
Add a micromotor that drives the gear to the iris adjustment ring of the manual iris fixed lens, and lead the 3 or 4 core shielded wire from the drive circuit to the camera's auto iris interface socket. When the luminous flux entering the lens changes, the charge generated by the camera CCD target surface changes accordingly, so that the level of the video signal changes, a control signal is generated, and is transmitted to the automatic iris lens, so that the motor in the lens is made positive Rotate or reverse to complete the resize task.
Manual iris zoom lens
Variable focus, there is a focal length adjustment ring, you can adjust the lens focal length within a certain range, the variable ratio is generally 2 to 3 times, the focal length is generally 3.6 ~ 8mm. In practical applications, the market's market angle can be easily selected by manually adjusting the zoom ring of the lens. However, it is inconvenient to frequently adjust the zoom manually after the camera is installed. Therefore, after the completion of the project, the focal length of the manual zoom lens is generally seldom adjusted. Only play the role of prime lens.
Automatic aperture electric zoom lens
Compared to the automatic aperture prime lens, two micro motors are added, one of which
The zoom ring of the motor and the lens can control the focal length of the lens when it is rotated; the other motor is fused with the focus of the lens, and the focus of the lens can be achieved when it is controlled to rotate. However, as two motors are added and the number of lens groups is increased, the volume of the lens is also increased accordingly.
Electric three variable lens
Compared with the automatic iris motorized zoom lens, the control of the aperture adjustment motor is changed from automatic control to manual control by the controller.
Press the focal length
According to the size of the field of view:
Press the focal length
(about 50 degrees), wide-angle lens and extra wide-angle lens (100-120 degrees)
Standard lens
The angle of view is about 50 degrees, which is also the angle of sight that can be seen when the monocular eyes are not turning. The focal length of the standard lens of the 5mm camera is mostly 40mm, 50mm or 55mm. 120 The standard lens focal length of the camera is mostly 80mm or 75mm. The larger the CCD chip, the longer the focal length of the standard lens.
Wide-angle lens
The viewing angle of 90 degrees or more is suitable for shooting scenes with a large distance and a wide range of scenes. It can also deliberately exaggerate the strong sense of foregroundness and perspective. The typical wide-angle lens of the 35mm camera is a focal length of 28mm and a viewing angle of 72 degrees. The camera's 50, 40mm lens is equivalent to a 35mm, 35mm, 28mm lens.
Long focal length lens
Suitable for shooting distant subjects, the small depth of field makes it easy to highlight the main body of the background, but it is bulky and difficult to focus on the dynamic subject. 35mm camera long focal length lens is usually divided into three levels, 135mm below the middle focal length, 135-500mm said long focal length, 500mm or more called ultra-long focal length. The camera's 150mm lens is equivalent to a 35mm camera's 105mm lens. Due to the long focal length of the lens is too heavy, so there is a telephoto lens design, that is, plus a negative lens behind the lens, the main plane of the lens forward, can be used to obtain a shorter lens body to obtain a long focal length effect.
Reflective telescope head
Another super telescopic lens design uses reflective mirrors to form images. However, because of the design relationship, it is impossible to mount an aperture. Only the shutter can be used to adjust the exposure.
marco lens
In addition to very close-up macro photography, it can also be telephoto.
By interface
C type lens
The flange focal length is the distance between the installation flange and the point of convergence of the incident light parallel to the lens. The focal length of the flange is 17.526mm or 0.690in. Install ribs as: diameter 1in, 32 teeth.in. The lens can be used with a linear array sensor with a length of 0.512in (13mm). However, due to geometric distortion and market angular characteristics, it must be identified whether the short-focus lens is suitable for use. Lenses with a focal length of 12.6 mm should not use line arrays longer than 6.5 mm. If the distance from the lens to the array is determined using the flange focal length dimension, then the lens attachment ring needs to be added when the object magnification is less than 20 times. The circle is added behind the lens to increase the distance from the lens to the image, thinking that the focus range of most lenses is 5-10%. The long lens distance is the focal length/object magnification.
CS lens
With a 5 mm adapter ring, a C lens can be used on a CS-mount camera.
U-type lens
A zoom lens with a 47.526 mm or 1.7913 in flange focal length and a M42 x 1 rib. Designed primarily for 35mm photo applications (such as domestic and imported 135 camera lenses), it can be used in any array less than 1.25in (38.1mm) in length. It is recommended not to use a short focal length lens.
Special lens
Such as microscopic magnification system. Pay special attention to the difference between CS and C. To connect different types of cameras and different types of Len, you need to customize the adapter ring. Very expensive abroad, one appointment is better than one's own processing. The main parameters of the optical lens and evaluation The main parameters are focal length, field of view, object distance, aperture, shutter and so on. For the end of the lens
There is no better evaluation than MTF (Modulation Transfer Function). However, due to aberrations (for reasons of calibration), each range of the lens has an MTF value. These ranges refer to: (1) the near-axis portion, (2) the off-axis portion, and (3) when the optical system is asymmetrically distorted, the sub-portions of the above two portions in different directions. Each part has its own corresponding MTF value for different radiation energy wavelength ranges. MTF is the most commonly used and best index for evaluating imaging systems, and it is also the optimal index for guiding the integration of machine vision systems.
parameter
Focus and aperture
Depth of field: The range of moderately clear focus around the subject plays a crucial role in the appearance of the final image. In order to make full use of all apertures provided on the lens, fix the camera on a tripod to prevent camera shake.
f/number of apertures and aperture size
Set any kind of lens at a f/f-number to be able to transmit images of almost the same amount of light, because the aperture diameter is directly related to the focal length, for example, an 80 mm lens uses a 5 mm aperture diameter The aperture must be adjusted to f/16. Therefore, after the focal length of the lens is divided by the aperture diameter, the corresponding f/f-number is obtained.
Focal length marking
Adjust the focus ring thread. The lens extends from the camera. With the rotation of the focus ring, you can find the focal length you are adjusting by pointing at the mark on the fixed reference symbol.
Aperture adjustment
Rotate the aperture ring up to the next f/float (for example, from f/4 to f/5.6), reduce the aperture size by half (ie, reduce the amount of light on the film by half); rotate the aperture ring down to the next f/stop ( For example from f/4 to f/2.8). Double the aperture size.
Depth of field
As the lens focuses on the subject, the f/number of the aperture that corresponds to (or is close to) the focus can be found on both sides of the fixed reference symbol, and the relative depth under the focus mark can be identified to determine the effective depth of field.
The role of depth of field
Aperture Size Change: Focusing on a subject at the same distance through a lens with the same focal length shows how the adjustment of the aperture size changes the depth of field. In general, the foreground of the subject is enlarged by 1/3, and the depth of field is increased by 2/3. The smaller the aperture, the greater the depth of field. The depth of field of the F/2 aperture is far less than the depth of field of the f/16 aperture.
The distance from the subject to the camera:
Even with the same focal length and aperture, the depth of field is somewhat limited by the distance from the subject to the camera. The closer the subject is to the camera, the smaller the depth of field. The depth of field produced by focusing the lens at 15 feet (4.5 meters) is much greater than the depth of field generated by the lens at 5 feet (1.5 meters).
The change of the lens:
In the case of the same object distance and aperture, the use of lenses with different focal lengths can change the depth of field. The shorter the focal length of the lens is, the deeper and deeper it is. For super wide-angle lenses (8---15 mm), the depth of field is so large that no focusing is required because The depth of field for each level of aperture is clear.
Application area
Optical industrial lenses are widely used for object detection with extremely high reflectance, such as scratch detection on surfaces such as metal, glass, film, and wafers, chip and silicon wafer breakage detection, MARK point positioning, glass cutting machines, and dispensing Machine, SMT inspection, plate-making machine and other industrial precision alignment, positioning, parts identification, size measurement, industrial microscopy and other CCD visual alignment, measuring devices and other fields.
other
Magnification problem:
Optical lens magnification problem, many customers may not know or understand the lens magnification, resulting in the wrong operation of the lens or the purchase of less than the right lens, so, Pumus optical to this problem for professional Said, I hope to help you better understand the lens parameters.
Amplification rate Optical magnification
Image size relative to the magnification of the object
β=y’/y
=b/a
=NA/NA’
=CCDCamera element size / actual size of field of view
Electronic magnification
The electronic magnification is the magnification of the image presented on the CCD imaged by the camera.
Display magnification
The monitor magnification is the magnification of the object being displayed on the monitor through the lens imaging.
Display magnification = (optical magnification) × (electronic magnification)
Example: Optical magnification = 0.2X, CCD size 1/2 (diagonal length 8mm), display 14 inches
Electronic magnification = 14 × 25.4/8 = 44.45 (times)
Display magnification = 0.2 × 44.45 = 8.89 (times) (1 inch = 25.4mm)
Field of view
The field of view is the range of objects that can be seen when the lens is connected to the CCD camera
The size of the field of view is: (CCD format size)/(optical magnification)
Example: Optical magnification = 0.2X, CCD 1/2 inch (4.8mm long, 6.4mm wide)
Field of view : Length=4.8/0.2=24(mm)
Width = 6.4/0.2 = 32 (mm)
