Note that the greatest cosmetic improvement on lens thickness (and weight) benefits from choosing a frame that holds physically small lenses. The smallest of the popular adult lens sizes available in retail outlets is about across. There are a few adult sizes of , and although they are quite rare, can reduce lens weight to about half of the 50 mm versions. The curves on the front and back of a lens are ideally formed with the specific radius of a sphere. This radius is set by the lens designer based on the prescription and cosmetic consideration. Selecting a smaller lens will mean less of this sphere surface is represented by the lens surface, meaning the lens will have a thinner edge (myopia) or center (hyperopia). A thinner edge reduces light entering into the edge, reducing an additional source of internal reflections.