The focal length of the lens can be understood from a practical perspective as the distance from the center of the lens to the film plane. The theoretical definition is: the vertical distance from the optical center of the lens or lens group to the focal plane when an infinite scene is formed into a clear image on the focal plane through a lens or lens group. For a fixed-focus lens, the position of its optical center is fixed; for a zoom lens, the change of the optical center of the lens brings about a change in the focal length of the lens.

The focal length of the modern 135 camera lens varies from 6mm to 2000mm, and the commonly used focal length ranges from 15mm to 600mm. For cameras with the same frame, facing the same subject, the change in imaging effect brought about by the change of lens focal length can be summarized into the following two laws:

1. The focal length of the lens is inversely proportional to the angle of view. A long focal length and a small viewing angle means that a larger scene can be captured at a long distance; a short focal length and a large viewing angle mean that a wide range of scenery can be captured at a close distance.

2. The focal length of the lens is inversely proportional to the depth of field. Long focal length and small depth of field mean that the clear range of front and back scenes are small; short focal length and large depth of field mean that the clear range of front and back scenes is large.

The lens has two fingers in film and television, one refers to the optical components used by movie cameras and projectors to generate images, and is composed of multiple lenses. Various lenses have different styling characteristics. Their application in photographic styling constitutes an optical expression method; the second refers to a continuous picture taken from power-on to power-off, or a segment between two splicing points , Also called a ytterbium. One finger and two fingers are two completely different concepts. In order to distinguish the difference between the two, one refers to the optical lens and the two refers to the lens image.

The lens referred to in film and television is not a lens in a physical or optical sense, but a lens that carries an image and can form a picture.

The lens is the basic unit that composes the entire film. Several shots constitute a paragraph or scene, and several paragraphs or scenes constitute a film. Therefore, the lens is also the basic unit of visual language. It is the basis of narrative and ideology. In the pre-shooting of film and television works, the lens refers to the sum of a section of pictures taken by the camera from start to standstill; in the post-editing, the lens is a set of pictures between two editing points; in the finished film, one The lens refers to the complete segment from the previous optical transition to the next optical transition.

The main function of the lens is to collect the reflected light from the illuminated object and focus it on the CCD. The image projected on the CCD is inverted. The camera circuit has the function of reversing it, and its imaging principle is the same as that of the human eye.