Normally light is focussed by the cornea and the lens to form a sharp image on the Retina. Sometimes the eye ball is too long and the light comes to a focus before the retina. Lenses to correct this are thicker at the edges than in the middle. In a stong prescription, the edges can be particularly thick and unsightly. There are a number of ways to avoid this. Choosing a small frame reduces the edge thickness and weight of the lens. Lenses with a high “refractive index” also reduce the edge thickness and weight. The third way is to use aspheric lenses which reduce the thickness further still. Anti-reflection coatings can also improve the appearance and vision through the lenses.