What are the characteristics of optical lenses?

Optical lens is an important part of TV camera, which is generally composed of convex lens, concave lens and corresponding metal parts. At present, the general camera lens has automatic aperture, electric zoom and other devices Optical lens is the portal of the camera, and its most basic function is to image the subject on the photosensitive element in the camera. The optical characteristics of 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 expressed and distinguished by the technical parameters of these three optical characteristics.

For TV photographers, lens focal length, field of view angle and relative aperture will all affect the picture shooting, and their technical performance and combination relationship directly determine the technical and artistic possibilities that photographers can achieve.

First, the focal length

The lens of a camera can be regarded as a convex lens with a thick center and a thin edge. The light passes through the lens and converges into a focus. The distance from the focal point to the center of the lens is the focal length of the lens, and the unit of focal length is millimeters (mm).

The length of the focal length of the lens, the charge coupled device of the object being photographed.

The imaging area on (CCD) is proportional. If you shoot the same subject at the same distance, the longer the focal length of the lens, the larger the imaging area and 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 CCD imaging area of a general professional camera is approximately equal to the frame image plane of 16 milli-water film camera, and the focal length of a standard lens is usually 25mm. A lens with a focal length greater than the diagonal of the image plane is called a long focal length lens, and a lens with a focal length less than the diagonal of the image plane is called a wide-angle lens. A lens whose focal length can be changed is called a zoom lens.

Second, the perspective.

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 CCD and the rear node of the lens.

From the modeling point of view, the lens field of view reflects the openness of the scene recorded by the camera (the lens field of view is divided into horizontal field of view and vertical field of view. The perspective used in this chapter refers to the horizontal perspective). 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 angle, the smaller the imaging of the subject and the wider the scene. Conversely, the smaller the field of view angle, the larger the imaging of the subject and the narrower the field of view of the scene.

The viewing angle is mainly restricted by two factors: the imaging size of the lens and the focal length of the lens. Because the imaging target surface of the camera tube is a constant fixed factor in actual shooting, the focal length of the lens directly affects the field of view. When shooting, we can only change the angle of view by changing lenses with different focal lengths.

When the camera shoots the same object at the same distance, using lenses with different focal lengths will change the imaging area and background uniformity of the object in the picture. This is essentially due to the corresponding changes in perspective. For example, the subject photographed with a lens with a 50-degree field of view is only110 of the image area photographed with a lens with a 5-degree field of view.

. The longer the focal length of the lens, the smaller the field of view angle; The shorter the focal length, the larger the field of view angle. What we usually call a standard lens (25mm lens) is that the focal length is about equal to the diagonal length of the imaging plane, and the horizontal field of view angle is about 45; For the zoom lens on the camera, that is, the lens with a focal length of about 25mm, the horizontal field of view angle of the wide-angle lens (focal length less than 25 mm) is greater than 60, and one lens is at 60-

130. Lenses above 130 to 180 are called super wide-angle lenses, also known as fisheye lenses, and the horizontal field of view of telephoto lenses (focal length greater than 25mm) is less than 40.