I want to ask about the composition of the camera.

A camera generally consists of a camera unit, a video recording unit, an image viewfinder, a microphone and accessories. Its internal structure is shown in Figure 5- 1. All cameras work on the same principle: converting optical images into electronic signals. The specific process is as follows: the scene is focused on the "target surface" of the camera equipment through the lens group. The "target surface" is a kind of light guide material, which can convert optical images into electrical signals according to the brightness of imaging, and send them to the video recording unit after circuit processing; A microphone receives sound signals and converts them into electrical signals. After passing through a sound processing circuit, it is recorded on a medium such as a magnetic tape together with an image signal.

camera lens

The lens of a camera (1ens), like the lens of a camera, is composed of several groups of lenses, and its main function is to focus the light reflected by the subject on the imaging element. Generally, a hood (1ens hood) is installed in front of the lens of a professional camera to prevent stray light from forming halo on the lens surface and affecting the image quality; Secondly, it helps to protect the lens when handling the camera. Lens can be divided into fixed-focus lens or fixed-focus lens and zoom lens. Fixed focal length lens can be divided into ordinary lens, telephoto lens and short focal length lens (or wide-angle lens). Zoom lens is a combination of these two types of lenses, which can be continuously changed between different focal lengths as needed. The ratio of the longest focal length to the shortest focal length of a zoom lens is the zoom multiple.

focal distance

Focal length refers to the distance from the optical center of the lens to the focal point of the lens image.

The focal length is a basic characteristic of the lens, which can determine the magnification of the image and the horizontal viewing angle of the lens. The shorter the focal length, the wider the horizontal viewing angle and the smaller the image. The size, proportion and sense of distance of the scene shot by standard lens are the closest to the scene directly seen by human eyes. Short-focus lens (wide-angle lens) is smaller and farther than the standard lens, but it has a wide visual range and a large viewing angle. A long focal length lens (telescope head) can zoom in and out of distant scenes, but the viewing angle is small. The imaging effects of two different lenses are shown in Figure 5-2.

Therefore, the focal length determines the width of the visual threshold of a specific camera. For zoom lens, the lens can continuously change from the maximum visual threshold to the minimum visual threshold, and the visual threshold is inversely proportional to the change of focal length, that is, it decreases with the increase of focal length and increases with the decrease of focal length. The imaging of an object changes positively with the change of focal length, that is, it becomes larger with the increase of focal length and smaller with the decrease of focal length. The zoom lens can start from any focal length and continuously change the focal length of the lens at any speed, so that the imaging and visual thresholds can be continuously changed. Figure 5-3 shows the imaging effects of three kinds of focal length lenses.

concentrate

When the light is converged on the screen of the camera tube through the lens group and accurately imaged, it shows that the focal length of the camera imaging has been adjusted. Because the distance between the subject and the lens changes at any time, the focal length of the lens must be adjusted at any time to ensure accurate imaging. The front lens group is used for focusing, and the focal length can be adjusted by rotating its outer ring. There are two kinds of focal length adjustment, manual and automatic, which can be selected by the control keys on the machine. The most basic requirement for zoom lens is that the brightness and clarity of the image will not change when zooming. All lenses (zoom and fixed focus lenses) have a minimum shooting distance (the shortest distance allowed between the subject and the lens, at which a clearly focused image can still be obtained). The lens with shorter focal length has shorter shooting distance than the lens with longer focal length.

hole

In addition to the focus ring, there is also a rotating ring on the lens, which controls the aperture of the camera lens and determines the amount of light entering the lens. When the external light is too strong, the aperture should be reduced appropriately; When the light is too weak, the aperture should be increased appropriately. Its purpose is to keep the intensity of light passing through the lens stable, so that the obtained image will not be too bright or too dark, and maintain a suitable gray level. Aperture has a set of adjustable apertures (diaphragm or iris), which can be opened or narrowed to control exposure. The size of the aperture is determined by the aperture coefficient (aperture number). We can see the numbers representing the aperture coefficient (1.4, 2, 2.8, 4, 5.6, 8,1,16,22) on the aperture ring. These seemingly unrelated coefficients. In this way, each aperture coefficient represents half or twice the light quantity of its left and right adjacent aperture coefficients. In addition, the aperture coefficient is inversely proportional to the aperture (iFiverse relation.ship). When the aperture coefficient is 8, the optical input is half of that when the aperture coefficient is 5.6, and the aperture coefficient is twice of that when the aperture coefficient is 1 1.

The lens is graded according to its minimum aperture coefficient (maximum aperture value)-maximum light input. A lens with an aperture coefficient of. 1.4 belongs to fastlens (short exposure time) because its aperture can let a lot of light pass through in unit time. Factory/4 lens is slow: the lens, that is, the exposure time is long, because its aperture allows much less light to pass through per unit time.

depth of field

Depth of field means that the scene before and after the focus can keep a clear distance (or range). The depth of field is determined by the size of the aperture, the focal length of the lens and the distance between the subject and the camera. The greater the focal length of the lens, the shallower the depth of field. Therefore, the depth of field of a wide-angle lens (short-focus lens) is larger than that of a telescopic lens (long-focus lens). Generally speaking, the closer the subject is to the camera, the shallower the depth of field.

If two subjects are distributed in a straight line with the camera, and their distances from the camera are different, in order to make both images as clear as possible, we have to set the focus in the middle of them, not in the center, and generally set the focus at half the distance from the former (subjects close to the camera), that is, the range of depth of field accounts for about half before the focus and about two-thirds after the focus, which is what we usually call the principle of half depth of field.

The focal length affects the depth of field, because the natural depth of field of wide-angle lens is larger than that of telephoto lens, and people design wide-angle lens to shoot large-scale and deep scenes. The smaller the aperture (the larger the aperture coefficient), the deeper the depth of field. We can use the characteristics of depth of field to make the whole picture clear only by the subject, and the rest are in a fuzzy state, so that we can highlight the subject. Many lens manufacturers and professional publications, such as the Manual of American Cinematographers, have compiled the depth of field of lenses with different focal lengths in various situations into tables for reference.

Camera unit

The function of the camera unit is to convert the optical signal entering the lens into an electrical signal, and then after processing by various circuits, an electrical signal is finally obtained, which is called a video signal.

Camera equipment

The camera device can be a camera tube or a ccD semiconductor chip. The image formed by the external scene through the lens just falls on the photosensitive surface of the camera device. There are many small photosensitive cells, called pixels, arranged on the photosensitive surface, and each pixel can convert the perceived light into electrical signals. The more pixels per unit area, the stronger the resolution of the image and the higher the definition of the image. We call the ability to distinguish images resolution, which is expressed by the number of black and white lines that can be distinguished in the horizontal direction of the whole picture.

Each pixel of the camera device will generate an image electrical signal corresponding to the photographed object, which contains various information such as brightness, contrast and chromaticity. The brightness of an image refers to the brightness of the whole image; Contrast refers to the contrast (or black-and-white contrast) between bright and dark parts in an image; Chromaticity includes hue and saturation, where hue represents the color of the image and saturation represents the depth of the color. All these electrical signals are sent to the circuit for processing.

Signal processing circuit

There are many links in image signal processing. This paper mainly introduces gain, white balance and digitization.

(1) gain

Gain is the amplification of the signal by the circuit. The electrical signal sent by the camera is very weak after photoelectric conversion, so it must be amplified to a standard value through the circuit before it can be sent to the video recorder and monitor. The size of the signal varies with the brightness of the subject. When shooting in a dark place, you can't get a normal image even if the aperture is opened to the maximum, so you need to increase the gain. The unit of gain is decibel (dB). Every time the signal is doubled, the gain is increased by 6 decibels. The normal gain is 0 dB, and the gains are generally divided into +6 dB, +9 dB,+12 dB,+18 dB, +24 dB, etc., which should be selected according to different environments. The larger the gain range of the camera, the higher the sensitivity and the stronger the adaptability to the dark environment. The camera sensitivity index indicates the lowest ambient illumination with appropriate image brightness when the camera is at the maximum gain, and the unit is lux (1x). The greater the gain, the noise in the circuit will be amplified at the same time, which shows that the clutter increases, the particles become thicker, the signal-to-noise ratio (the ratio of useful signal to clutter) decreases, and the image quality is damaged.

(2) White balance

The white balance circuit is an important link in the image signal processing circuit, which is directly related to the accuracy of image color reproduction. White balance is one of the commonly used terms in photography. To understand the concept of white balance, we must start with light and color.

Light is electromagnetic wave, and the wavelength of visible light is in the range of 380~780 nm (nanometer). Light waves with different wavelengths will not only cause bright feeling (brightness), but also cause different color feeling (hue) in human eyes. When the wavelength changes from long to short, the colors perceived by human eyes are red, orange, yellow, green, cyan, blue and purple in turn, which are collectively called spectrum. White light is the color perception produced by the mixed action of monochromatic light with different wavelengths on human eyes. In daily life, the color of the scenery we see is not only related to its own physical characteristics, but also closely related to the light source that illuminates it. When an object is illuminated by light, it can absorb some wavelengths of light and reflect or transmit other wavelengths of light. The color feeling produced by this reflected or transmitted light acting on our eyes is the color of the object. For example, in sunlight, safflower can reflect red light and absorb other colors of light, so it appears red; White objects can reflect various wavelengths of light, so they look white. If a white object is illuminated by red light, it will appear red. This shows that the hue of the light source will affect the human eye's perception of the color of the object, so it is necessary to choose a suitable light source to reproduce the color of the object correctly.

The hue of a light source is usually expressed by its color temperature. Burning "absolute blackbody radiator" (such as a closed carbon block that never reflects incident light) will emit different colors of light at different temperatures. When a certain type of light source and an absolute blackbody radiator have the same characteristics at a certain temperature, this specific temperature is defined as the color temperature of the light source, expressed in Kelvin, which is a thermodynamic absolute temperature scale with the unit of K, and the K on the Kelvin scale is -273℃ in degrees Celsius. Any light source can be represented by color temperature. The following table lists the color temperatures of several typical light sources:

Color temperature table unit of several typical light sources: K candle 1 390 tungsten filament incandescent lamp 3 000 iodine tungsten lamp 3 200 mercury lamp 4 500-5 500 fluorescent lamp 6 000 rainy day 7 000 sunrise and sunset 2 000-3 000 foggy day 8 000 noon sunshine 5 000-5 400 clear sky 10 000.

It can be seen that the color temperature of the light source is low, and its light tone is red; The color temperature of the light source is high, and its light is blue. In order to reproduce the color of the scene correctly, it is necessary to control the color temperature of the light source.

When the signal shot by the camera is restored to an image again, it should still reflect the original color, which is color restoration. Natural objects are colorful. How does the camera handle such rich colors? In fact, the video signal generated by a color camera is equal to the luminance signal generated by a black-and-white camera superimposed on the chrominance signal. Color has two basic elements: hue, that is, the unique hue of color (such as yellow, brown, red, etc. ), and saturation, that is, the concentration or purity of color (such as dark blue with high saturation, light blue with low saturation, etc. ). The brightness signal can represent the brightness level of the whole picture from the darkest part to the brightest part, and at the same time, it will affect the brightness of the color, making the color look bright or dark.

3CCD camera divides the light passing through the lens into two primary colors (red, green and blue)-each CCD is responsible for one color. Light passes through a prism or a special dichroic mirror and is decomposed into two primary colors according to the proportion of various colors in the scene (it is found that light of any color can be decomposed into three basic colors: red, green and blue). Finally, these color signals are reprocessed, and the white balance circuit is actually a circuit that can adjust these three primary color electrical signals.

Camera lens prism red target surface green target surface blue target surface output chromaticity signal Figure 5-4 Principle of chromaticity signal processing process

If the combined recording method is adopted, the chrominance signal and the luminance signal are recorded together; If the component recording method is adopted, the chrominance signal and the luminance signal are recorded separately.

(3) Digitization

Digital processing of image signals is the current development trend. Using analog-to-digital conversion circuit to digitize analog signals before transmission, processing and storage has many advantages, such as strong anti-interference ability, good stability, low loss, easy component integration, convenient large-scale rapid storage, and convenient online processing by computer. Digitalization is the basic condition for camera to improve performance and add new functions (such as digital zoom, oil painting, strobe, still frame effect, etc.). ).

Control circuit

The control circuit is used to control various functions of the camera. Some functions are automatically controlled, such as autofocus, automatic aperture, etc. Usually, automatic control is to detect the deviation state through the detection circuit, generate an error voltage through comparison and calculation, and then send it to the control circuit to correct the deviation state to the normal state. Some functions are manually controlled, and should be operated according to the actual situation when using.

Video recording unit

The video recording unit is a video recorder, and its function is to convert the video signal sent by the video processing circuit and the audio signal sent by the audio processing circuit into magnetic signals and record them on the magnetic tape. It can also be used as a video player. The video recording device consists of mechanical system (magnetic tape cassette, magnetic head and tape deck) and circuit system (recording and playback circuit, servo circuit and control circuit).