What are the optical characteristics of lenses? What are the factors? What influence do they have on the picture modeling?

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.

(1) 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 focal length of the lens is proportional to the imaging area of the object on the photoelectric target surface of the camera tube. 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 imaging area of photoelectric target surface of general camera is about equal to the frame image plane of 16mm film camera, and the focal length of 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. 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.

(2) perspective

The field of view angle of the lens refers to the included angle formed by the edge of the effective imaging plane (field of view) of the camera tube 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, and 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 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 our field of vision 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 range 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. The standard lens (25mm lens) presents a field angle of about 45. Wide-angle lenses (focal length less than 25mm) have horizontal field of view angles greater than 60, generally between 60-130. Lenses above 130 to 180 are called super wide-angle lenses, also known as fisheye lenses. The horizontal field of view angle of a long focal length lens (focal length greater than 25 mm) is less than 40.

(3) The relative aperture of the lens refers to the ratio of the diameter (d) of the incident light hole to the focal length (f) of the lens, and its size indicates how much light the lens receives. The relative aperture is an important factor to determine the light transmission ability and resolution of the 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, such as 1.4, 2, 2.8, 4, 5.6, 8,1,12, 16, 22, etc. The ratio of aperture f values of two adjacent stages is equal, and the exposure is different by one stage. Because the illumination of the image surface is proportional to the square of the relative aperture, the f value changes by one step, which is equivalent to twice the luminous flux of the camera lens. When shooting, when we say opening a large aperture, we actually move from a large f value to a small f value in the aperture adjustment ring, that is, reduce the aperture coefficient value; When the aperture decreases, it moves from a small f value to a large f value, and the aperture coefficient value increases. For example, when the aperture is adjusted from 8 to 5.6, the aperture becomes larger, the luminous flux is doubled, and the exposure value is increased by one level. or vice versa, Dallas to the auditorium

The adjustment of relative aperture and aperture coefficient determines the luminous flux and depth of field of the lens. The choice of camera lens aperture is essentially a problem of exposure control. Nowadays, cameras usually have two control modes: manual aperture and automatic aperture. Automatic aperture can only deal with the exposure control of the photographed scene technically, 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 wider the depth of field. The intentional control of lens exposure and the selective use of different depth of field are effective means for photographers to realize their creative intentions and obtain the best picture effect.