Lens features of the lens

The optical lens is an important component of the TV camera. It is generally composed of multiple convex lenses and concave lenses and corresponding metal parts. Nowadays, general camera lenses are equipped with automatic aperture, electric zoom and other devices. The optical lens is the gateway of the camera. Its most basic function is to image the subject on the photosensitive element in the camera. The optical properties of a lens refer to the physical properties formed by its optical structure, which are composed of three factors: 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 television camera operators, the focal length, field of view and relative aperture of the lens will all have an impact on the picture shooting. Their technical performance and composition The matching relationship directly determines the technical possibilities and artistic possibilities that the photographer can achieve. 1. Focal length The lens of a camera can be regarded as a convex lens with a thick middle and thin edges. Light passes through the lens and converges into a focus. The distance from the focus to the center of the lens is the focal length of the lens. The unit of focal length is millimeters (mm). . The focal length of the lens is directly proportional to the imaging area on the charge-coupled device (CCD) where the subject is stored. If you shoot the same subject at the same distance, the longer the focal length of the lens. Then the larger the imaging area, the higher the magnification; conversely, the shorter the focal length of the lens, the smaller the imaging area and the lower the magnification. Usually, we call a lens whose focal length is close to or equal to the diagonal of the image plane a standard lens. The imaging area of ??a charge-coupled device in a general professional camera is approximately equal to the image plane of a 16mm film camera, and the standard lens focal length is usually 25mm. A lens with a focal length greater than the diagonal of the image plane is called a telephoto lens, and a lens with a focal length smaller than the diagonal of the image plane is called a wide-angle lens. A lens whose focal length can change is called a zoom lens. 2. Field of view angle The field of view angle of the lens refers to the angle formed by the edge of the effective imaging plane (field of view) of the charge-coupled device and the node behind the lens. From a modeling perspective, the lens field of view reflects the openness of the camera's recording range (the lens' field of view is divided into horizontal field of view and vertical field of view. The field of view used in this chapter refers to the horizontal field of view). The field of view of the lens is inversely proportional to the imaging effect of the subject in the picture. The larger the field of view, the smaller the image of the subject, and the wider the picture. On the contrary, the smaller the field of view, the larger the image of the subject. , the narrower the field of view of the scene in the picture. The field of view is mainly restricted by the two factors of lens imaging size and lens focal length. Since the imaging target surface of the camera tube is a constant factor in actual shooting, the focal length of the lens directly affects the field of view. When we shoot, we generally can only change the angle of view by changing lenses with different focal lengths. When the camera shoots the same subject at the same distance, using lenses with different focal lengths will change the imaging area and background range of the object in the picture. This is essentially due to the corresponding change in the field of view. For example, the subject captured by a lens with a field of view of 50° is only 1/10 of the image area captured by a lens with a field of view of 5°. The longer the focal length of the lens, the smaller the field of view; the shorter the focal length, the larger the field of view. What we usually call a standard lens (25mm lens) is a lens with a focal length approximately equal to the diagonal length of the imaging plane and a horizontal field of view of about 45°; for a zoom lens on a camera, it is a lens with a focal length of about 25mm. The horizontal field of view of wide-angle lenses (focal length less than 25mm) is greater than 60°, and generally ranges between 60°-130°. Lenses between 130° and 180° are called ultra-wide-angle lenses, also known as fisheye lenses. The horizontal field of view of long-focal-length lenses (focal length greater than 25mm) is less than 40°. 3. Relative aperture and aperture coefficient The relative aperture of a lens refers to the ratio of the incident aperture diameter (D) of the lens to the focal length (f). Its size indicates how much light the lens accepts. Relative aperture is an important factor in determining the light transmittance and discrimination of a lens. The reciprocal of the relative aperture (D/F) (f/D) is called the aperture coefficient (f) and is marked on the aperture ring of the lens. The aperture coefficient of the camera lens is divided into several levels, the common ones are 1.4, 2, 2.8, 4, 5.6, 8, 11, 12, 16, 22, etc. The ratio of the F value of the two adjacent apertures is 2, and the exposure difference is one class. Since the image plane illumination is proportional to the square of the relative aperture, a change in F value by one stop is equivalent to a doubling of the luminous flux of the camera lens. When shooting TV pictures, when we say that we open a wide aperture, it actually moves from the large F value to the small F value on the aperture adjustment ring, that is, the aperture coefficient value is reduced; while narrowing the aperture, it moves from the small F value to the large F value. If one end of the value is moved, the aperture coefficient value increases. For example, adjusting from aperture 8 to aperture 5.6 means opening the aperture, doubling the light flux, and increasing the exposure value by one step, and vice versa. Adjustment of relative aperture and aperture coefficient determines the luminous flux and depth of field of the lens. Selecting the aperture of the camera lens is essentially a matter of exposure control. Today's cameras usually have two control methods: manual aperture and automatic aperture.

Automatic aperture can only perform technical processing on the exposure control of the scene being photographed, while conscious and purposeful dynamic lighting and artistic processing can only be better expressed by manual aperture. When shooting the same scene under the same illumination, the larger the aperture, the smaller the depth of field range; the smaller the aperture, the larger the depth of field range. Intentional control of lens exposure and selective use of different depths of field are effective means for TV crews to achieve their creative intentions and achieve the best picture effects. To sum up, focal length, field of view angle and relative aperture (aperture), the three parameters that represent the optical characteristics of the lens, are related to and restrict each other. They all have a direct impact on the shape of the picture. Lenses with different focal lengths, field of view and relative apertures can record very different pictures and their modeling effects. They have prepared a technical foundation for television photographers and provided creative ideas. convenience conditions. Among these three factors, the change in the focal length of the lens has the greatest impact on the shape of the picture. The most prominent effect during actual shooting is the change in the focal length of the lens. Therefore, if you want to do a good job in television photography, you must understand and master the picture modeling characteristics presented by different focal length lenses, and fully realize that optical lenses are not only a technical means, but also an artistic means, so as to be able to use them in television picture photography creation activities. Focus on the strengths and avoid the weaknesses, and give full play to the best picture modeling effects that can be obtained by lenses with different focal lengths.

This article comes from Pumis Optics