The focal length measures the strength of a lens. Consider a converging lens. Rays from a very distant object come together at the focal point. The focal length is the distance from the lens to the focal point. Lenses with very convex surfaces have shorter focal lengths, while flatter lenses have longer focal lengths. For diverging lenses the focal point is virtual. That is, it is the point from which the diverging rays leaving the lens would have come from if a point object were placed there.