The word "aspheric" means "not spherical."So, an aspheric lens is not shaped like a ball. The contours of the front surface change from the center of the lens to the edges. Aspheric lenses are designed to guide light rays through the lens so that they all focus together on the retina, and this design results in flatter lenses. Aspheric lenses are lighter, since there is less mass, making eyewear more comfortable. The aspheric design is available in CR-39, mid index 1.54, 1.56, 1.57 and polycarbonate (index 1.59). It is also available in high index 1.60, 1.66 and 1.70. A high index aspheric lens will provide the flattest, thinnest and lightest lens available with superior optics.