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The characteristics of optical lens


Optical lens is an important part of TV and camera, which is generally composed of multiple convex and concave lenses and corresponding metal parts. Now, the general camera lens is equipped with automatic aperture, electric zoom and other devices. Optical lens is the portal of the camera, and its basic function is to image the object on the sensitive element in the camera. The optical properties of lens are the physical properties formed by its optical structure, which are composed of focal length, field of view angle and relative aperture. Any kind of optical lens can be represented and distinguished by the technical parameters of these three optical characteristics. For TV photographers, lens focal length, field angle and relative aperture will have an impact on the picture shooting. Their technical performance and composition directly determine the technical possibility and artistic possibility that the camera can achieve.
1、 Focal length
The lens of the camera can be seen as a convex lens with thick middle and thin edges. The light will gather into focus through the lens. The distance from the focus to the lens center is the focal length of the lens, and the focal length unit is mm (mm). The length of lens focal length is proportional to the imaging area on the CCD. If the same subject is photographed at the same distance, the longer the lens focal length is, the larger the imaging area is, the higher the magnification is; On the contrary, the shorter the focal length of the lens, the smaller the imaging area and the lower the magnification.
Usually, we call a standard lens that has a focal length close to or equal to the diagonal of the image plane. The imaging area of charge coupler is about 16 mm. The standard lens focal length of the picture plane of a movie camera is usually 25 mm. Lens with focal length larger than the diagonal of image plane is called long focal distance lens and lens with focal length less than the diagonal of image plane, and is called wide angle lens. Lens with variable focal length is called zoom lens.
2、 Field of view angle
The field of view angle of a lens refers to the angle formed by the effective imaging flat (field) edge of the charge coupled device and the node behind the lens. From the perspective of modeling, the angle of view reflects the wide range of the camera recorded objects (the lens field angle is divided into horizontal field of view angle and vertical field of view angle. The field of view angle used in this chapter refers to the horizontal field of view angle). The contrast between the field of view angle and the imaging effect of the subject in the picture is inverse. The larger the field of view angle, the smaller the imaging of the subject, the wider the picture scene; On the contrary, the smaller the field angle, the larger the subject image, the narrower the field of view of the scene.
The field of view angle is mainly restricted by the size of the lens imaging and the focal length of the lens. Because the target surface of the camera tube imaging is a constant fixed factor in actual shooting, the focus of the lens is the direct factor that directly affects the field of view angle. When we shoot, we can only change the field of view angle by changing the lens with different focal length.
When the camera takes pictures of the same subject at the same distance, the lens with different focal length will change the imaging area and background of the object in the picture. This is essentially due to the corresponding changes in the field of view angle. For example, a field of view angle is 50 ° The subject captured by the lens of is only 5 in the picture ° The lens of the image takes one tenth of the area. The longer the focal length of the lens, the smaller the angle of view; The shorter the focal length, the larger the field of view angle. The standard lens (25mm lens) we usually call is that the focal length is approximately equal to the diagonal length of the imaging surface, and the horizontal field of view angle is 45 ° about; For zoom lens on camera, it is the lens with focal length of about 25mm. The horizontal field of view angle of wide angle lens (focal length less than 25mm) is greater than 60 °, One in 60 °-  one hundred and thirty ° between. one hundred and thirty ° Up to 180 ° The lens between is called the super wide angle lens, also known as fish eye lens. The horizontal field angle of view of the long focal distance lens (focal length is more than 25mm) is less than 40 °。

 3、 Relative aperture and aperture coefficient
The relative aperture of a lens refers to the ratio of the diameter of the incident light hole (d) to the focal length (f), and its size indicates how much light the lens receives. Relative aperture is an important factor to determine the transparency and discrimination of lens. The reciprocal (f/d) of the relative aperture (d/f) is called the aperture coefficient (f), which is marked on the aperture ring of the lens. The aperture coefficient of camera lens is divided into several grades, the common ones are 1.4, 2, 2.8, 4, 5.6, 8, 11, 12, 16, 22, etc. the ratio of F value of the adjacent two stages is 2, and the exposure amount is different by one stage. Because the illumination of image plane is proportional to the square of relative aperture, the F value changes one gear, which is equivalent to twice the change of the luminous flux of the camera lens. When shooting TV pictures, we say that opening a large aperture actually moves from the large F value on the aperture adjusting ring to the end of the small F value, that is, the aperture system value is reduced; The smaller aperture moves from small f to larger f, and the aperture system value increases. For example, when the aperture 8 is adjusted to aperture 5.6, the aperture is opened, the luminous flux increases by one time, the exposure value increases by one level, and vice versa.
The adjustment of relative aperture and aperture coefficient determines the luminous flux and depth of field of the lens. The choice of aperture for camera lens is essentially an exposure control problem. Nowadays, there are two control modes: manual aperture and automatic aperture. The automatic aperture can only deal with the exposure control of the scene, but the conscious and purposeful dynamic light and art processing can only be better performed by manual aperture. When shooting the same scene under the same illumination, the larger the aperture, the smaller the depth of field; The smaller the aperture, the larger the depth of field. The intentional control of lens exposure and the selective application of different depth of field are effective means for TV filmmakers to achieve the best picture effect.
In conclusion, the three parameters of lens optical characteristics, namely focal length, field of view angle and relative aperture (aperture), are related and restricted. They all directly constitute the influence on the image shape. The pictures and their modeling effects recorded by different focal length, field of view angle and relative aperture lens are very different. They provide the technical basis for TV photographers and provide the creation convenience. Among these three factors, the most important factor is the change of lens focus.

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