What does 1.665 aspherical lens mean?

It means aspherical lens. Aspherical lens refers to a lens whose surface is not spherical or cylindrical. In photography, a lens containing aspherical optical elements. The surface curvature of aspherical lenses is different from that of ordinary spherical lenses. In order to pursue lens thinness, the curved surface of the lens needs to be changed. In the past, spherical designs were used, which increased aberrations and deformation, resulting in obvious unclear images, distorted vision, and poor visual field. Narrow and other undesirable phenomena.