Editing Corrective lens (section)

=== Reducing lens thickness ===
[[File:Specs Lens Thickness 2.jpg|thumb|right|400px|Crude relationship between lens size and its thickness for the same radius of curvature. In addition to its smaller surface area, the small lens is also much thinner and so is much lighter.]]
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 {{convert|50|mm|abbr=on}} across. There are a few adult sizes of {{convert|40|mm|abbr=on}}, and although they are quite rare, can reduce lens weight to about half of the 50&nbsp;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.

Extremely thick lenses for myopia can be [[beveled]] to reduce flaring out of the very thick edge. Thick myopic lenses are not usually mounted in wire frames, because the thin wire contrasts against the thick lens, to make its thickness much more obvious to others.

The index can improve the lens thinness, but at a point, no more improvement will be realized. For example, if an index and lens size is selected with center to edge thickness difference of 1&nbsp;mm then changing index can only improve thickness by a fraction of this. This is also true with aspheric design lenses.

The lens's minimum thickness can also be varied. The FDA ball drop test (5/8" 0.56-ounce steel ball dropped from 50 inches)<ref>Code of Federal Regulations Title 21 "Food and Drugs" Sec. 801.410 [http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/CFRSearch.cfm?fr=801.410 ''Use of impact-resistant lenses in eyeglasses and sunglasses'']</ref> effectively sets the minimum thickness of materials. Glass or CR-39 requires 2.0&nbsp;mm, but some newer materials only require 1.5&nbsp;mm or even 1.0&nbsp;mm minimum thickness.