What is the focal length of a standard lens?

A standard lens usually refers to a photographic lens with a focal length of 40 to 55 mm. The perspective of the scene represented by the standard lens is close to the vision, and it is the most basic photographic lens among all lenses. Lens focal length refers to the distance from the main point to the focal point after lens optics, which is an important performance index of lens. The length of the focal length of the lens determines the imaging size, viewing angle, depth of field and viewing angle of the picture.