Ask what a lens is.

The lens is a structure that focuses light, so that the film can get a clear image. Early lenses were all composed of a convex lens. Because of the poor definition, chromatic aberration will occur, which is gradually improved into a compound lens, that is, the combination of multiple concave-convex lenses to correct various aberrations or chromatic aberration. By coating the lens, the amount of light entering is increased, glare is reduced, and the image quality is greatly improved. Generally speaking, the lens used for photography is a focusing lens. According to the optical principle, the light from far away will all focus on one point after passing through the lens with focusing function, which is the focus. The distance from the focal point to the center of the lens is called the focal length. On a camera, the center of the lens is usually at the aperture and the focus is at the focal plane (i.e. the film surface). So the focal length of the camera is the distance from the aperture to the film when the lens is focused at infinity. Lens type (classified according to the application) Wide-angle lens: the viewing angle is above 90 degrees, the observation range is large, and the close-up image is distorted. Standard lens: the viewing angle is about 30 degrees, and it has a wide range of use. Telephoto lens: The viewing angle is less than 20 degrees, and the focal length can reach tens or hundreds of millimeters. Zoom lens: the focal length of the lens is continuously variable, and the focal length can be changed from wide angle to telephoto. The longer the focal length, the larger the imaging. Pinhole lens: used for hidden observation, often installed on the ceiling or wall. ]