Input (grade 13): A far sharper image can be obtained using materials with a high refractive index, decreasing to the edges - this decreases the focal length and thus allows a sharp image to form on the retina.
Output (grade 9): It is possible to get a better image with materials that have a high refractive index which decreases towards the edges. This decreases the focal length and allows a sharp image to form on the retina.