Basic knowledge of photographic optical lens

Basic knowledge of photographic optical lens

Optical lens is an indispensable part of machine vision system, which directly affects the imaging quality and the realization and effect of the algorithm. The following is the basic knowledge of photographic optical lens that I share with you. Welcome to read and browse.

1 Introduction

For cameras, the quality of the lens has always been a key factor affecting the imaging quality, and digital cameras are no exception. Although the CCD resolution of digital camera is limited, the optical resolution of lens is low in principle; On the other hand, because the imaging area of digital camera is small (because digital camera images on CCD, which is much smaller than the film of traditional 35mm camera), lens is needed to ensure a certain imaging quality. For example, a subject needs 200 pixels in the horizontal direction to perfectly reproduce its details. If the imaging width is 10mm, a lens with an optical resolution of 20 lines /mm is enough. If the imaging width is 1mm, the optical resolution of the lens must be above 2000 lines /mm. On the other hand, traditional film is sensitive to ultraviolet rays, so it is often necessary to install a UV mirror when shooting outdoors, while CCD is sensitive to infrared rays. Adding a special coating or filter to the lens will greatly improve the imaging quality. The physical aperture of the lens must also be considered, and regardless of its relative aperture, the larger its physical aperture, the greater the luminous flux, the better the digital camera accepts and controls the light, and the better the imaging quality.

Some manufacturers use relatively good lenses for commercial or household digital cameras. Fuji camera adopts professional Fuji Dragon lens with the resolution of 170 lines/mm. This brand-new built-in Fuji Dragon lens is clearer than most SLR lenses. Not only does it ensure the quality of image shooting in accuracy, but its lens error rate also reaches an astonishing 0.3%, which is 2/3 lower than that of ordinary digital cameras.

In addition, in some digital cameras, there are also two lens modes: telephoto and wide angle. This is also a reference index when you choose a digital camera.

In traditional digital cameras, wide-angle lens is a photographic lens with shorter focal length, larger viewing angle, longer distance and smaller viewing angle than standard lens. Wide-angle lenses are divided into ordinary wide-angle lenses and ultra-wide-angle lenses. 135 camera's common wide-angle lens has a focal length of 38-24 mm and a viewing angle of 60-84 degrees. The ultra wide-angle lens has a focal length of 20- 13mm and a viewing angle of 94- 1 18 degrees. Because the wide-angle lens has a short focal length and a large viewing angle, it can shoot a large area of scenery in a short shooting distance. Therefore, it is widely used to shoot large-scale landscape photography. In photography creation, using wide-angle lens to shoot can achieve the following effects: first, it can increase the sense of spatial depth of photographic images; Second, the depth of field is long, which can ensure that the scene before and after the subject can be clearly reproduced on the screen. Therefore, the vast majority of modern pocket automatic cameras (commonly known as fool cameras) use ordinary wide-angle lenses of 38-35 mm; Third, the lens covers a large area and shoots a wide range of scenery; Fourthly, the scene shot at the same shooting distance is smaller than the scene image shot with the standard lens; Fifth, perspective distortion and image distortion are easy to appear in the picture. The shorter the focal length of the lens and the closer the shooting distance, the more obvious this defect is.

Commercial digital cameras often use the same general wide-angle lens as ordinary 35 mm cameras. A digital camera with the same performance will perform better when choosing a digital camera because of its deep scene and wide shooting range.

2 classification

By structure

Fixed aperture fixed focal length 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

Compared with the fixed-aperture fixed-focus lens, the manual aperture fixed-focus lens adds an aperture adjusting ring, and the aperture range is generally from F 1.2 or F 1.4 to fully closed, which can easily adapt to the illumination of the shooting scene. Aperture adjustment is done manually. The illumination is relatively uniform and the price is relatively cheap.

Automatic aperture fixed focus lens

A gear-driven micro-motor is added to the aperture adjusting ring of manual aperture fixed-focus lens, and a 3-core or 4-core shielded wire is drawn from the driving circuit and connected 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

If 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. The zoom ratio 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 the automatic aperture fixed focus lens, two micro motors are added, one of which is connected with the zoom ring of the lens, which can control the focal length of the lens 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.

Electric tri-variable lens

Compared with automatic aperture electric zoom lens, it only changes the control of aperture adjusting motor from automatic control to manual control of controller.

focus

According to the size of the field of view: small field of view lens, ordinary lens (about 50 degrees), wide-angle lens, ultra-wide-angle lens (100- 120 degrees).

standard lens

The viewing angle is about 50 degrees, which is also the viewing angle that a person can see with one eye without turning his head and eyes, so it is also called standard lens. The standard lens focal length of a 5mm camera is mostly 40mm, 50mm or 555 mm.. The standard lens focal length of 120 camera is usually 80mm or 75mm. The larger the CCD chip, the longer the focal length of the standard lens.

wide-angle lens

The viewing angle is greater than 90 degrees, which is suitable for shooting scenes with short distance and large range, and the foreground can be deliberately exaggerated to show a strong sense of distance, that is, perspective. A typical wide-angle lens of a 35mm camera has a focal length of 28mm and a viewing angle of 72 degrees. The 50,40mm lens of 120 camera is equivalent to the 35,28mm lens of 35mm camera.

telephoto lens

Suitable for shooting distant view, small depth of field is easy to make the blurred background subject stand out, but it is huge and difficult to focus on the dynamic subject. The long focal length lens of a 35mm camera is usually divided into three levels:1below 35mm is the medium focal length, 135-500mm is the long focal length, and 500mm is the long focal length. 150mm lens of 120 camera is equivalent to 105mm lens of 35mm camera. Because the lens with long focal length is too heavy, there is a design of telescope head, which is to add a negative lens behind the lens and move the main plane of the lens forward to get the lens body with short focal length.

Reflecting telescope tou

It is another design of telescope head, which uses mirror to image, but because of design reasons, it is impossible to install aperture, so the exposure can only be adjusted by shutter.

macro lens

In addition to taking extremely close macro photography, you can also take long-distance photography.

Media interface

C-lens

The focal length of the flange is the distance from the mounting flange to the convergence point of parallel light incident on the lens. The flange focal length is 17.526mm or 0.690in. Mounting rib: diameter 1in, 32 teeth. The lens can be used for linear sensors with a length of 0.5 12 inch (13mm). However, due to geometric deformation and market angle characteristics, it is necessary to distinguish whether the short-focus lens is suitable or not. For example, a lens with a focal length of 12.6mm should not use a linear array with a length greater than 6.5 mm If the distance between the lens and the array is determined by the focal length of the flange, then when the magnification of the object is less than 20 times, a lens connecting ring should be added. A connecting ring is added behind the lens to increase the distance from the lens to the image, which means that the focus range of most lenses is 5- 10%. The lens contact distance is the focal length/object magnification.

CS lens

With the 5 mm adapter ring, the C lens can be used on the CS bayonet camera.

U-shaped lens

Zoom lens, flange focal length 47.526mm or 1.79 13in, mounting rib M42? 1。 It is mainly designed for 35mm camera applications (such as domestic and imported 135 camera lenses) and can be used for any array with a length less than 1.25in(38. 1mm). Short focal length lenses are not recommended.

Special lens

Such as a microscopic amplification system. Pay special attention to the difference between CS and C, and customize the adapter ring when connecting different types of cameras and different types of lenses. It's expensive abroad, so it's better to process it yourself. The main parameters and evaluation parameters of optical lens are focal length, field of view, object distance, aperture, shutter and so on. For the end of the lens

The evaluation of goodness is nothing more than MTF (modulation transfer function). However, due to aberration (calibration reasons), each range of the lens has an MTF value. These ranges refer to: (1) the near-axis part, (2) the off-axis part, and (3) the sub-parts of the above two parts in different directions when there is asymmetric distortion in the optical system. For different wavelength ranges of radiation energy, each part has its corresponding MTF value. 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.

Three parameters

Focus and aperture depth of field: A moderately clear focus range around the subject plays an important role in the appearance of the final image. In order to make full use of all the apertures provided on the lens, the camera can be fixed on a tripod to prevent the camera from shaking.

F/ aperture number and aperture size

Any kind of lens with a certain f/ aperture number can transmit images with almost the same amount of light, because the aperture diameter is directly related to the focal length. For example, when an 80 mm lens uses a 5 mm aperture diameter, the aperture must be adjusted to f/ 16. So after the focal length of the lens is divided by the aperture diameter, the corresponding f/ aperture number is obtained.

Focus mark

Adjust the thread of the focus ring so that the lens protrudes from the camera. As the focus ring rotates, you can find the adjusted focus by placing marks aligned with fixed reference symbols.

Aperture adjustment

Turn the aperture ring up to the next f/ aperture number (for example, from f/4 to f/5.6), and the aperture size will be halved (that is, the light quantity of the film will be halved); Lower the aperture ring to the next f/ aperture number (for example, from f/4 to f/2.8). Double the aperture size.

Depth of field range

With the lens focusing on the subject, we can find the f/ number corresponding to (or close to) the set aperture on both sides of the fixed reference symbol, identify the relative value under the focal length mark, and then determine the effective depth of field.

The role of depth of field

The change of aperture size: By focusing the lens with the same focal length on the subject with the same distance, it shows how the adjustment of aperture size changes the depth of field. Generally speaking, the subject's foreground depth of field is enlarged by 1/3, and the back depth of field is enlarged by 2/3. The smaller the aperture, the greater the depth of field. The depth of field of F/2 aperture is much smaller than that of f/ 16 aperture.

Distance from the subject to the camera: Even if the same focal length and aperture are used, the depth of field is limited to some extent by the distance from the subject to the camera. The closer the subject is to the camera, the smaller the depth of field. Focusing at 15 feet (4.5 meters) produces a much larger depth of field than focusing at 5 feet (1.5 meters).

Lens change: When the object distance and aperture are the same, the depth of field can be changed by using lenses with different focal lengths. The shorter the focal length of the lens, the greater the depth. For a super wide-angle lens (8- 15 mm), the depth of field is too large to need focusing, because the depth of field of each aperture is very clear.

4 application

Optical industrial lenses are widely used in the positioning detection of high-reflectivity objects, such as scratch detection of surfaces such as metals, glass, films and wafers, damage detection of chips and silicon wafers, marking point positioning, industrial precision alignment, positioning, part confirmation, size measurement, industrial microscope and other CCD visual alignment and measuring equipment.

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