What is the principle of optical imaging with high-definition camera? The finer the description, the better.

First, the working principle of the camera

A camera is a device that converts a light image of a scene into an electrical signal. Its structure can be roughly divided into three parts: optical system (mainly lens), photoelectric conversion system (mainly camera tube or solid-state camera device) and circuit system (mainly video processing circuit).

The main component of an optical system is an optical lens, which consists of a lens system. This lens system contains many lenses with different convexity and concavity, in which the middle of the convex lens is thicker than the edge, so the light passing through the edge of the lens will be refracted more than the light passing through the central part. When the object is refracted by the lens of the optical system, it forms a "focus" on the imaging surface of the camera tube or solid-state camera device of the photoelectric conversion system. The photosensitive element in the photoelectric conversion system will convert the optical image outside the "focus" into an electric signal with charge. These electrical signals are weak, and they must be further amplified by the circuit system to form signals that meet specific technical requirements and output from the camera.

The optical system is equivalent to the camera's eyes, which is closely related to the operation skills, and will be introduced in detail in the following chapters of this chapter. The photoelectric conversion system is the core of the camera, and the camera tube or solid-state camera device is the "heart" of the camera. The content of this part will be introduced in the third chapter. Since most household cameras combine the camera part and the video part into one, the working principle of the video part will be summarized below.

When the camera system in the camera converts the optical image of an object into a corresponding electrical signal, a recording signal source is formed. The video recording system converts the electric signal sent by the signal source into a magnetic signal through the electromagnetic conversion system and records it on the video tape. If you need the camera's playback system to play back the recorded signal, you can manipulate the relevant buttons to turn the magnetic signal on the video tape into an electrical signal, which is amplified and sent to the TV screen for imaging.

From the perspective of energy conversion, the working principle of the camera is a light-electricity-magnetism-electricity-light conversion process.

Second, the lens and its imaging principle

It is the most important part of the camera and is called the human eye. The human eye can see everything in the universe because it can form an image on the retina by the physical ability of the eyeball crystal; The camera can shoot and image. It mainly relies on the lens to form the image of the object to be photographed and project it onto the imaging surface of the camera tube or solid-state camera device. So, the lens is the eye of the camera. The expressive ability of TV pictures, such as clarity and richness of image levels, is restricted by the inherent quality of optical lenses. At present, the lenses of various cameras that are common in the market are coated lenses. Coating is to coat a layer of color film on the lens surface to reduce the dispersion phenomenon between lenses, reduce the glare caused by backlight shooting, protect the light from passing through the lens smoothly, improve the light transmission ability of the lens and make the shot picture clearer.

In the process of self-study photography, photographers should first be familiar with the imaging principle of lenses, including focal length, angle of view, field of view and image field.

Focal length is short for focal length. For example, when a magnifying glass faces the sun on one side and the paper on the other, it moves up and down to a certain distance, and the paper will gather into a bright spot, which will soon burn the paper into a small hole, so it is called "focusing". The distance from the center of the lens to the paper is the focal length of the lens. For a camera, the focal length is equivalent to the distance from the "center" of the lens to the imaging surface of the camera tube or solid-state camera device.

Focal length is one of the important data to indicate the performance of optical lens, because the size of the image taken by the lens is controlled by focal length. In the process of shooting TV cameras, photographers often change the focal length for modeling and composition to form a variety of visual effects. For example, when shooting the same target at the same distance, the longer the focal length of the lens, the narrower the horizontal viewing angle of the lens and the smaller the shooting range of the scene; The shorter the focal length of the lens, the wider the horizontal angle of view of the lens, and the larger the range of the scene.

The range of scenery covered by camera lens is usually expressed by angle, which is called lens angle. Included in the visible image formed by the object on the focal plane through the lens is the field of view of the lens. However, the clarity and brightness of the image presented in the field of view are different at the center and edge. The images near the central part and the central part are clear and bright. The image definition of the edge part is poor and much darker. The image of this edge part is not available for camera shooting. So when designing the camera lens, only the field of view is used. It should be pointed out that the size of the final photo taken by the camera does not depend entirely on the size of the image field of the lens. That is to say, the imaging size of the lens must be consistent with the optimal size of the imaging surface of the camera tube or solid-state imaging device.

When the imaging size of a camera lens is determined, for a lens with a fixed focal length, it has a fixed field of view, which is often used to indicate the size of the field of view. Its law is that the shorter the focal length, the larger the viewing angle and field of vision. So a short focal length lens is also called a wide-angle lens.

Third, the depth of field principle of the lens

When the lens focuses on a certain point of the object, the object at that point can be clearly imaged on the TV screen. Scenery in a certain range before and after this point can also be recorded more clearly. In other words, the clear range of the scene shot by the lens is limited. The depth range of the object that can be recorded "clearly" in front of the camera tube is the depth of field. When the lens is aimed at the subject, the clearance range in front of the subject is called foreground depth of field, and the clearance range in the back is called back depth of field. The foreground depth of field and the background depth of field together, that is, the depth of the whole TV picture from the nearest clear point to the farthest clear point, is called Quan Jingshen. Generally speaking, the depth of field refers to Quan Jingshen.

In some pictures, the front of the subject is clear and the back is blurred, in some pictures, the back of the subject is clear and the front is blurred, and in some pictures, only the subject is clear and blurred. These phenomena are all caused by the depth of field characteristics of the lens. It can be said that the principle of depth of field plays an extremely important role in camera shooting. Correct understanding and application of depth of field is helpful to take satisfactory photos. There are three main factors that determine the depth of field:

When the focal length of the aperture lens is the same and the shooting distance is the same, the smaller the aperture is, the larger the depth of field is. The larger the aperture, the smaller the depth of field. This is because the smaller the aperture, the thinner the beam entering the lens, the more obvious the paraxial effect and the smaller the angle of light convergence. In this way, the converging light in front of the imaging plane will leave a smaller spot on the imaging plane, so that the scenery that was not clear from the lens has acceptable clarity.

Focal length When the aperture coefficient and shooting distance are the same, the shorter the focal length of the lens, the greater the depth of field. The longer the lens focuses, the smaller the depth of field range. This is because the focal zone (focal depth) formed by the lens with short focal length is much narrower than that of the lens with long focal length, so more light spots will enter the acceptable definition area.

When the focal length and aperture coefficient of the lens are equal, the farther the object distance is, the wider the depth of field is. The closer the object distance, the smaller the depth of field range. This is because the scenery far from the lens can be clearly focused with a little adjustment, and the focus of the front and rear scenes is closely gathered. This will allow more light spots to enter the acceptable definition area, so the depth of field will increase. On the contrary, when focusing on a scene close to the lens, the distance between the front and rear focal points increases, that is, the focal depth range increases, so that the number of spots entering the acceptable definition area decreases and the depth of field decreases. For this reason, the foreground depth of a lens is always smaller than the depth of field behind it.

4. Zoom lens and its principle Camera lens can be divided into standard lens, long focal length lens and wide-angle lens. Take a 16mm camera as an example, the focal length of its standard lens is 25mm. The reason why this focal length is determined as the focal length of a standard lens is mainly because this focal length is similar to the normal horizontal viewing angle of human eyes (24 degrees). When shooting with a standard lens, the spatial and perspective relationship of the subject is the same as what the photographer sees in the viewfinder. A focal length greater than 50 mm is called a long focal length lens, and a focal length less than 16 mm is called a wide-angle lens. The standard of camera lens division is basically the same as that of 16 mm camera. But at present, most TV cameras in China only use a zoom lens, that is, a lens system can realize the continuous conversion from "wide-angle lens" to "standard lens" or even "telephoto lens", which brings great convenience to the shooting operation.

One of the main characteristics of telephoto lens is that it can change the focal length within a certain range without changing the imaging plane position, and it has become the most widely used lens in home cameras.

The zoom lens consists of many Dan Toujing. The simplest is a combination mirror composed of two convex lenses. Now, let the distance between the two lenses be X. Through practice, we can know that the focal length of the combined lens can be changed by changing the distance between the two convex lenses. This is the most basic principle of zoom lens. However, the disadvantage of the above combined lens is that when the distance of x changes, not only the focal length changes, but also the position of the imaging plane changes. In order to keep the position of the imaging plane unchanged, it is necessary to add several groups of lenses and move together regularly. Therefore, the zoom lens in the camera should have at least three groups of combined lenses, namely, focusing group, zooming group and image plane compensation group. If the image distance is too long and the brightness of the imaging surface is not right, and it is necessary to shorten the image distance, a group of combined lenses will be added, which is called the objective lens group. Fig. 5 is a structural diagram of a zoom lens.

When the zoom lens zooms, the viewing angle also changes, but the focus position and aperture opening remain unchanged. Usually, the focus multiple of a lens refers to the ratio of the longest focal length to the shortest focal length of a zoom lens. At present, in some popular cameras, the zoom range of the zoom lens is generally from 10-90 (mm), so its multiple is about 6-8 times. The zoom lens of some broadcast cameras is about 14- 15 times. In addition, some machines are equipped with zoom multipliers to double the focal length of the lens on the basis of the longest focal length, thus expanding the telephoto range of the lens. However, this zoom device will affect the image quality, so be careful when using it.

In actual shooting, when the zoom lens gradually changes from the wide-angle end to the telephoto end, the visual effect of the picture seems to be that the camera is getting closer and closer to the scene, which is the so-called "push lens". The opposite change effect is "pulling the lens". There are two control methods to change the zoom of camera lens, one is electric zoom, and the other is manual zoom. Electric zoom is controlled by an electric push-pull rod (T push -W pull), and the force exerted by the hand on the push-pull rod can change the moving speed of the lens. The characteristic of electric zoom is that the lens changes evenly during the push-pull process. Manual zoom is achieved by pulling the zoom ring directly by hand. Manual zoom is generally used when the lens needs to be pushed and pulled quickly.

The operation of zoom lens is more difficult, and beginners will find it even more difficult, because the factors that affect the focus definition, such as lens focal length, aperture, depth of field and the distance between the subject and the camera, may change at the same time. In order to solve this problem effectively, beginners can grasp this point when shooting, that is, focus on the subject with the longest focal length of the zoom lens first, and then return to the required focal length when shooting, so as to ensure the clarity of the subject.