What Is Focal Length?
The focal length, which is frequently specified in millimeters (mm), is the essential description of a camera lens. It is calculated from the optical distance from the focal plane of the camera's digital sensor or 35mm film to the point where light rays converge to form a sharp image of an object, rather than being a measurement of the actual length of the lens. It is possible to determine a lens' focal length when it is focused to infinity.
Both, the magnification and the angle of view, or how much of the scene will be caught, depend on the focal length of the lens. Magnification increases and the field of view narrows as the focal length increases. More expansive angles of view and lower magnifications result in a shorter focal length.