Principle, classification and development prospect of camera

The formation of photography and cameras

Photography is not only widely used in all fields of national economy, but also has become an indispensable part of people's modern civilized life.

The origin of modern photography can be traced back to the pinhole imaging principle mentioned by Mozi (468 ~ 376 BC) in Mo Jing and Zhao Youqin's pinhole imaging box in Yuan Dynasty. In Europe, leonardo da vinci, a famous painter in the16th century, found that a small hole was poked in the window of a room, and then all the doors and windows were closed, and the room was completely dark. At this time, the scenery outside the window can be clearly reflected on the indoor wall through the small hole. This is the principle of pinhole imaging in physics. Later, other painters hung white paper on the wall and traced it according to the reflected lines. When the painter moves the distance between the white paper hanging on the wall and the small hole, the image reflected on the white paper can be enlarged or reduced, which solves a major technical problem of drawing at that time.

From the end of 17 to the beginning of 18, with the development of glass industry, people made flat glass and glass lenses. Someone made a black box by using the principle of pinhole imaging in darkroom. Install convex lens instead of pinhole on the box, and install frosted flat glass on the other end of the box. The convex lens focuses the projected light, and people use brushes to draw various natural scenery on the flat glass. This black box is the most primitive camera. In order to improve the imaging quality, opticians constantly make a fuss about lenses and form a series of photographic lenses, which are called photographic objective lenses by modern people. Mechanical designers constantly improve and transform the heavy wooden camera box, which is what modern photographers call the camera body.

But it is too much trouble to take down the scenery reflected on the glass with a brush, so it is necessary to invent a kind of photographic paper that can be sensitive to light. 18 13 Niepce, France, found that a kind of asphalt would change color after being exposed to the sun, which had certain photosensitivity, so it was used as a photosensitizer. The specific method is: dissolving asphalt in peppermint oil to make a solution, and then coating it on a metal plate; After exposure, it was soaked in kerosene, so that peppermint oil was dissolved in kerosene, and the image appeared on the metal plate. But the image is still embarrassing. Later, French painter Daguerre and Hamps studied together. It was not until 1839 that Daguerre solved the technical problems of developing and fixing, and the world recognized that photography was invented.

At that time, the "film" was a silver iodide photosensitive plate, and its photosensitive performance was really poor. In addition, most cameras use a telephoto lens consisting of one or two lenses. The light entering the camera box is very weak, so it takes a long time to take a picture, and the formed image is too poor. People are determined to further improve the sensitivity of the photosensitive plate to light, that is, [url=] sensitivity [/url]. 187 1 The invented silver bromide gelatin dry printing method uses gelatin instead of nitrocellulose glue, and silver bromide instead of silver iodide, which is coated on a glass sheet to make a dry printing plate. This can greatly improve the sensitivity, and the exposure time can be shortened to a fraction of a second, a fraction of a second, or even shorter.

In order to adapt to the rapid improvement of photographic film sensitivity and control exposure time, people installed shutters in cameras. In this way, people can photograph fast-moving animals, such as birds and galloping horses. The embryonic form of a modern camera consists of a series of main components, such as lens, shutter, film and body, which are initially improved with the development of photography technology.

(2)- The basic composition of the camera

First of all, the lens

The lens focuses the scene on the reflection of the film. In order to make the images of objects in different positions clear, besides the lens itself, the aberration should be corrected, and the object distance and image distance should also maintain the yoke relationship. Because of this, the lens should be able to move back and forth to focus, so a better camera generally needs a focusing mechanism.

Second, the viewfinder.

In order to determine the scope of the subject and facilitate shooting and composition, the camera should be equipped with a viewfinder. The viewfinder of modern cameras also has the functions of ranging and focusing.

Third, the exposure control mechanism-shutter and aperture

In order to adapt to different subjects with different light and dark, and obtain correct exposure on film, it is necessary to control the exposure time and the intensity of light entering the lens. Therefore, the camera must set the shutter to control the exposure time, and set the aperture to control the light quantity by adjusting the aperture size.

Fourth, the film feeding counting mechanism

In order to prepare for the second shooting, the exposed film needs to be pulled away and the exposed film needs to be pulled over, so modern cameras need a film conveying mechanism. In order to display the number of films shot, a counting device is needed.

Five, the fuselage

Not only the [url=] black box [/url] of the camera, but also the combination of various components of the camera. The most basic components of a camera can be represented by a block diagram.

In fact, as far as the basic functions of the camera are concerned, the early "silver camera" is not much different from today's highly electronic, automated and computerized cameras.

3)- Classification of cameras

Generally, cameras can be classified according to their technical characteristics, such as frame size, framing mode, shutter mode, metering mode, etc., and can also be classified according to their appearance and structure. The specific classification is as follows:

(1) Depending on the film and frame size used by the camera.

It can be divided into 35mm camera (often called 135 camera), 120 camera,10 camera, 126 camera, medium camera and large camera. 135 camera uses 35mm film, and its standard frame is 24mm X 36mm. Generally, 36 or 24 shots can be taken per film.

(2) according to the appearance and structure of the camera

It can be divided into head-up camera and single-lens reflex camera. In addition, there are dual-lens reflex cameras, folding cameras, conveyors, landlines and so on.

(3) according to the shutter form of the camera

It can be divided into lens shutter camera (also known as center shutter camera), focal plane shutter camera and program shutter camera.

(4) According to the function and technical characteristics of the camera

It can be divided into auto-focus camera, manual exposure camera and automatic exposure camera. In addition, there are shutter priority, aperture priority, program control, double priority, electric film winding (automatic film winding and rewinding) camera, automatic focus (AF) camera, date back camera, built-in flash camera and so on.

Sometimes it can be divided according to the purpose of the camera, such as one-step imaging camera and stereo camera; Sometimes it can also be divided into zoom or bifocal cameras according to the characteristics of the lens. In fact, a modern camera often has many characteristics, so it should be defined comprehensively.

(4)- fundamentals of photographic optics

Generally speaking, the working process of a camera is to use the principle of optical imaging to image an object on a photosensitive material through a photographic lens. The following will briefly introduce the principle of photographic optical imaging: human understanding of the nature of light, the propagation of light and the principle of lens imaging.

Human understanding of the nature of light has gone through a long and tortuous process. In the whole18th century, the particle flow theory of light is still dominant in optics. It is generally believed that light is composed of tiny particles, which are emitted from point light sources and radiate in all directions in a straight line. /kloc-At the beginning of the 9th century, represented by the works of Jan and Fresnel, it gradually developed into today's wave optical system. Now, the understanding of the essence of light is that light, like physical objects, is a kind of substance, which has both the properties of wave and particle (quantum), but as a whole, it is neither wave nor particle, nor a mixture of the two.

In essence, light is no different from ordinary radio waves. Like electromagnetic waves, light is a shear wave, that is, the vibration direction of the wave is perpendicular to the propagation direction. Luminous body is the emission source of electromagnetic wave, and the electromagnetic wave emitted by luminous body propagates to the surrounding space, similar to the wave generated by water wave fluctuation. The distance between two points with maximum or minimum intensity is called wavelength, which is denoted by λ. The time required to propagate a wavelength is called a period, which is represented by t, and the period is the time required for a particle to complete a vibration. The number of vibrations in 1 sec is called frequency, which is expressed by nu. The distance that vibration travels through 1s is called velocity, which is represented by "v". There are the following relationships among wavelength, frequency, period and speed:

v=λ/T，ν= 1/T，v=λν

It can be seen that the wavelength of light is inversely proportional to the frequency. In fact, light waves only account for a small part of the entire electromagnetic wave band. The electromagnetic wave with the wavelength of 400 ~ 700 nm can be felt by human eyes, which is called visible light. Beyond this range, the human eye will not feel it. Different wavelengths of visible light produce different color sensations in our eyes. According to the wavelength from long to short, the color of light is red, orange, yellow, green, cyan, blue and purple. The propagation speed of electromagnetic waves with different wavelengths in vacuum is exactly the same, and the value is c = 300000 km/s.

Since light is an electromagnetic wave, studying the propagation of light should be a wave propagation problem. However, when designing optical instruments such as camera lenses, light is not regarded as an electromagnetic wave, but as a geometric line that can spread energy, called light. The light emitted by the light source A emits numerous geometric lines to the surroundings, and these numerous directional geometric lines are called rays. In this way, the study of light propagation in geometric optics becomes a geometric problem and a mathematical problem, and the problem is much simplified.

Here are some basic laws of geometric optics-the law of light propagation:

(1) The law of linear propagation of light in a uniform medium, that is, light travels along a straight line, that is, light is a straight line in a uniform medium. The linear propagation of light can be seen at any time and place in daily life, such as objects becoming shadows after being irradiated by light, [url=] pinhole [/url] imaging and so on. The linear propagation of light leads to the concept of light.

(2) The Law of Independent Propagation of Light The propagation of light is independent. When different rays pass through a certain point in the medium from different directions, they do not affect each other. When two rays converge at a certain point in space, its function is simply superposition. This characteristic of light makes the light from all points of the object enter the camera lens without affecting each other, forming an image on the imaging plane.

(3) Law of Reflection of Light When light propagates to the interface of two different media, it will change the propagation direction and reflect the light. The law of light reflection states:

(1) The normals of incident light, reflected light and light projection points on the interface are in the same plane, and the incident light and reflected light are located on both sides of the normal respectively.

② The shooting angle and reflection angle are equal. The angle between the incident light and the normal n is recorded as the incident angle, which is expressed by I; The angle between the reflected light and the normal n is recorded as the reflection angle, which is expressed by α. Then i=α. The reflection of light is also reversible. If light is incident on the interface against the original reflected light, it will be reflected against the original incident light. According to different interfaces, reflection can be divided into directional reflection and diffuse reflection. Light is incident on a bright plane mirror from one direction, all incident points fall on the same plane, and all reflections are in the same direction, which is called directional reflection. When light is projected from one direction onto a rough surface (such as ground glass surface), because the rough surface can be regarded as composed of many facets with different angles, the light is reflected from different directions, which is called diffuse reflection. However, it should be noted that in the diffuse reflection phenomenon, every ray still follows the reflection law.

The reflection of light plays a very important role in photography. For example, people themselves do not emit light, but when light shines on people from all angles, it can be reflected from all angles. We often use reflected light to take pictures, which is to follow the reflection law of light.

(5)- Characteristics and classification of photographic lenses

Photographic lens is one of the most important parts of a camera, which generally consists of a plurality of positive lenses, negative lenses, cemented lens groups, and metal gaskets and lens barrels for fixing these optical elements. Its function is to clearly image the subject on [url=] photosensitive [/url] film.

First, the optical characteristics of photographic lens

The optical characteristics of photographic lens can be expressed by three parameters, namely, focal length f, relative aperture D/f and field of view angle 2ω. In fact, as far as 135 camera is concerned, its standard frame has been determined to be 24mm X 36mm, and its diagonal length is 2η=43.266. There is the following relationship between the focal length f of the camera lens and the viewing angle ω:

tgω=η/f

Where: 2η- diagonal length of frame;

F- the focal length of the lens.

Another most important optical characteristic index of camera lens is the relative aperture. It indicates the ability of the lens to pass light, expressed by D/F, and defined as the ratio of the aperture diameter (also called the entrance pupil diameter) D of the lens to the focal length F of the lens (Figure 1-2-9). For example, if the maximum aperture diameter of a camera lens is 25mm and the focal length is 50mm, then the maximum relative aperture of this camera lens is 1/2. The reciprocal of the relative aperture is called the aperture coefficient or aperture number of the lens, also called the f number, that is, f = F = F=f/D D d.

The lens of a camera should be marked with the aperture number. According to the size of luminous flux, the national standard stipulates that the order of aperture numbers at all levels is 0.7, L, 1.4, 2, 2.8, 4, 5.6, 8, 1 1, 16, 22 .. However, the national standard allows the maximum relative lens. When the focal length f is fixed, the number of f is inversely proportional to the entrance pupil diameter d, and the light passing area is directly proportional to the square of d, so the larger the light passing area, the greater the luminous flux that the lens can pass. Therefore, when the aperture number is the smallest, the aperture is the largest and the luminous flux is the largest. With the increase of aperture number, the aperture becomes smaller and the luminous flux decreases. Every difference in aperture (its numerical ratio is 1.4 14) will double its luminous flux. If we don't consider the influence of the difference of transmittance of various lenses, as long as the aperture value is the same, their luminous flux is the same no matter how long the focal length of the lens is and how large the aperture diameter of the lens is. F number is a particularly important parameter for camera lens. The smaller the F number, the wider the scope of application of the lens.

Second, the classification of photographic lens

There are many ways to classify photographic lenses, but they are usually classified as follows:

(l) According to the focal length or field angle of the lens, the lens can be divided into three categories: standard lens, short focal length (wide angle) lens and telephoto lens.

When general cameras are sold, most of them are equipped with standard lenses. The focal length of the standard lens is basically equal to the diagonal length of the negative frame. Although there are still differences in the field of view (generally between 45 and 55), most of them are close to the human eye's perspective. Therefore, the perspective relationship of pictures taken with standard lens is more in line with people's visual habits. Due to the moderate focal length, field of view, shooting range and depth of field of the standard lens, and the image size obtained at the same shooting distance, this lens is the most widely used and is most suitable for taking pictures of portraits, landscapes, life and so on.

A wide-angle lens is a short-focus lens. According to the length of the focal length, there are wide-angle and ultra-wide-angle lenses. Its characteristics are short focal length, large field of view and wide shooting range. When the distance cannot be increased in a narrow environment, the wide-angle lens can expand the shooting field of vision and take photos of panoramic or large scenes within a limited distance. The wide-angle lens also has the characteristics of super-scale rendering, which exaggerates the prospect. In photography, we can make full use of the special perspective created by it to exaggerate the depth of the scene and highlight the main part. The wide-angle lens has short focal length and long depth of field, and the photos taken are very clear. Therefore, it is suitable for capturing some activities that are too late to focus calmly, and it is more suitable for taking news photos of big scenes or photos of indoor family life. Wide-angle lens is an indispensable photographic lens in landscape photography because of its large wishing angle and wide depth of field. At present, ordinary plastic automatic cameras on the market are equipped with wide-angle lenses.

Medium focal length lens belongs to long focal length lens. The focal length of a medium focal length lens is about twice that of a standard lens, and the focal length of a long focal length lens is longer. Its * * * features are: long focal length, small field of view and large imaging on the negative. Therefore, at the same distance, you can take larger images than the standard lens. It is suitable for photographing the activities of distant people or animals, and shooting some inaccessible objects to get natural and vivid pictures. Because the range of depth of field of the medium-long focal length lens is smaller than that of the standard lens, it is beneficial to blur the background of the subject, and the subject is generally far away from the camera, which has less deformation in the perspective of the portrait or the main scene and will make the portrait more vivid. Therefore, people often call the medium focus lens a portrait lens. Ordinary civilian users rarely use telephoto lenses, because the lens barrel of telephoto lenses is long and heavy, relatively expensive, and the depth of field is relatively small, so it is difficult to focus in practical use, so it is often used for professional photography.

(2) The lens can be divided into super-transparent lenses according to its condensing ability, and the relative aperture of the photographic objective should be above 1:2.8; Strong light transmission lens,1:3.5 ~1:5.8; Normal transparent lens,1:6.3 ~1:9; Weak light transmission lens, less than 1:9.

(3) According to whether the focal length of the lens can be changed, it can be divided into fixed-focus lens and zoom lens.

Due to the rapid improvement of optical design level and optical glass melting technology, it is possible for wealthy photographers to choose an optical system whose focal length can be changed within a certain range while keeping the image plane fixed. This kind of camera lens, which can change the focal length value in a certain range, so as to obtain different field of view angles, different images and different scene ranges, is called zoom photographic objective lens, or zoom lens for short. Zoom lens can change the shooting range by changing the focal length without changing the shooting distance, so it is very beneficial to picture composition. Because a zoom lens can also serve as several fixed-focus lenses, it not only reduces the number of photographic equipment to carry when traveling, but also saves the time for changing lenses. At present, almost all the top automatic fool cameras produced abroad are equipped with zoom lenses with small zoom ratio.

Zoom lenses can be divided into single ring and double ring according to different zoom methods. Single-ring zoom lens, zoom and focus with the same pull ring, push and pull it to zoom and rotate it to focus; Its advantages are simple and quick operation. Double-ring zoom lens, one of which is used for zooming and focusing respectively; The advantages are that zooming and focusing do not interfere with each other, and the accuracy is high, but the operation is more troublesome. At present, some zoom lenses on the market are marked with the word "micro" on the front ring of the lens, which means that they can be used for macro photography or ultra-close photography, so such zoom lenses are more widely used.

However, due to its complex optical system and mechanical structure, zoom lens is difficult to manufacture and is restricted by price, volume and weight. It is impossible to make the relative aperture of the zoom lens very large. Sometimes, in order to reduce the volume or ensure aberration, the lens can only change the aperture.

(6)- Aberration and lens grade

Influence of Aberration on Imaging Quality

Photographic lens grade standard

Due to the influence of many factors such as optical design, processing technology and assembly technology, it is impossible to form an ideal image of an object with a certain size. The actual image is always different from the ideal image, and this imaging difference is called aberration of the lens (or imaging optical system).

Aberration is caused by the physical conditions (optical characteristics) of the optical system. In a sense, any optical system has some aberrations, which can not be completely eliminated in theory. The naked eye and other light energy receivers only have a certain resolution, so as long as the aberration is less than a certain limit, we think that the aberration of the system has been corrected. Let's briefly introduce the aberration classification, formation and correction methods of photographic lenses.

The aberration of lens can be divided into two categories: monochromatic aberration and chromatic aberration.

First of all, monochromatic aberration

If the lens only images monochromatic light, then * * * has five kinds of aberrations with different properties. They are spherical aberration, coma, astigmatism, field curvature and distortion that affects the similarity between objects and images.

1, spherical aberration

After imaging, the light with a single wavelength emitted from the object point on the optical axis to the lens will not converge to the same point on the image side because of the different focusing abilities of each point on the lens spherical surface, but will form a symmetrical speckle centered on the optical axis. This aberration is called spherical aberration, and the size of spherical aberration is related to the position of the object point and the aperture angle of the imaging beam. When the position of the object point is determined, the smaller the aperture angle, the smaller the spherical aberration. With the increase of aperture angle, the increase of spherical aberration is proportional to the higher power of aperture angle. In a photographic lens, if the number of apertures increases by one step (the aperture decreases by one step), the spherical aberration will be reduced by half. Therefore, when shooting, as long as the light intensity allows, in order to reduce the influence of spherical aberration, a smaller aperture should be used to take pictures.

Step 2 be in a coma

An object point outside the optical axis emits a beam of parallel light to the lens, and after passing through the optical system, an asymmetric diffuse spot will be formed on the image plane. This diffuse light spot is comet-shaped, that is, a tail from thin to thick is dragged from the center to the edge, and its head end is bright and clear, and its tail end is wide, dim and awkward. The aberration caused by this off-axis beam is called coma. The size of coma is represented by the asymmetry of scattered light spots formed by coma. The size of coma is related to aperture and field of view. Like the spherical aberration in shooting, we can take appropriate measures to reduce the influence of coma on imaging.

In the field of photography, the modulus phenomenon caused by spherical aberration and coma is generally called halo. In most cases, the halo of off-axis points is larger than that of on-axis points. Due to the existence of off-axis aberration, our requirements for off-axis image points cannot be higher than those for on-axis image points, that is, they have the same imaging defects. At this time, we call it isochromatic imaging. With the increase of relative aperture, it will be more difficult to correct spherical aberration and coma. When using a large aperture lens, you should know the performance of the lens in advance, and pay attention to the minimum aperture. If possible, the aperture should be narrowed as much as possible to improve the imaging quality.

Step 3 Astigmatism

Astigmatism is also an off-axis image base, which is different from coma. It describes the aberration of imaging defects of infinite thin beams, which is only related to the field of view. Due to the asymmetry of the off-axis beam, the convergence point of the meridional beamlet at the off-axis point and the convergence point of the sagittal beamlet are in different positions, and the aberration corresponding to this phenomenon is called astigmatism. The projection of the distance between the convergence point of the meridian beamlet and the convergence point of the solitary vector beamlet on the optical axis is the value of astigmatism. Due to the existence of astigmatism, the image quality of off-axis field of view is obviously reduced, and even if the aperture is small, it is impossible to obtain a very clear image in both meridian and sagittal directions. The size of astigmatism is only related to the angle of view, and has nothing to do with the aperture size. Therefore, the astigmatism of wide-angle lens is obvious, so try to keep the subject in the center of the picture when shooting.

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When the object plane perpendicular to the optical axis passes through the optical system, it does not image on the same image plane, but on a curved surface symmetrical with the optical axis. This imaging defect is called field curvature. Field curvature is also an aberration independent of aperture. Due to the existence of astigmatism, the curved image plane formed by the meridian small beam and the curved image plane formed by the sagittal small beam often do not overlap, which are called meridian field curvature Xt and sagittal field curvature Xs respectively. When you take a picture with a curved lens, when the focus is focused on the center of the picture, the images around the picture will be blurred; When the focus is on the clear image around the picture, the image in the center of the picture begins to fade again, and it is impossible to get a clear image in the center and periphery on a flat image plane. Therefore, in some special [url=] cameras [/url], the negative is deliberately placed in an arc position to reduce the influence of field curvature. Because the field curvature of a wide-angle lens is always larger than that of a general lens, when taking a group photo, the imaging quality of the edge field of view is improved by arranging the subject into an arc.

Step 5 deform

Distortion refers to the deformation of the image formed by the object in shape. Distortion does not affect the clarity of the image, but only the similarity of the object image. Due to the existence of distortion, the straight line in the object space becomes a curve in the image side, which leads to image distortion. Distortion can be divided into barrel distortion and pillow distortion. Distortion has nothing to do with the relative aperture, but only with the field of view of the lens. Therefore, special attention should be paid to the influence of distortion when using wide-angle lenses.