Cosmetics and weight




Reducing lens thicknessedit

Note that the greatest cosmetic improvement on lens thickness (and weight) benefits from choosing a frame which holds physically small lenses. The smallest of the popular adult lens sizes available in retail outlets is about 50 mm (2.0 in) across. There are a few adult sizes of 40 mm (1.6 in), 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.

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.

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 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) effectively sets the minimum thickness of materials. Glass or CR-39 requires 2.0 mm, but some newer materials only require 1.5 mm or even 1.0 mm minimum thickness.

Weightedit

Material density typically increases as lens thickness is reduced by increasing index. There is also a minimum lens thickness required to support the lens shape. These factors result in a thinner lens which is not lighter than the original. There are lens materials with lower density at higher index which can result in a truly lighter lens. These materials can be found in a material property table. Reducing frame lens size will give the most noticeable improvement in weight for a given material. Ways to reduce the weight and thickness of corrective lenses, in approximate order of importance are these:

  • Choose glasses frames with small lenses; that is to say, so that the longest measurement across the lens at any angle is as short as possible. This gives the greatest advantage of all.
  • Choose a frame that allows the pupil to occupy the exact middle point of the lens.
  • Choose a lens as near round as possible. These are less commonly found than other shapes.
  • Choose as high a refractive index for the lens material as cost permits.

It is not always possible to follow the above points, because of the rarity of such frames, and the need for more pleasing appearance. However, these are the main factors to consider if ever it should become necessary and possible to do so.

Facial distortion and social stigmaedit

Eyeglasses for a high-diopter nearsighted or farsighted person cause a visible distortion of his or her face as seen by other people, in the apparent size of the eyes and facial features visible through the eyeglasses.

  • For extreme nearsightedness the eyes appear small and sunken into the face, and the sides of the skull can be visible through the lens. This gives the wearer the appearance of having a very large or fat head in contrast with their eyes.
  • For extreme farsightedness the eyes appear very large on the face, making the wearer's head seem too small.

Either situation can result in social stigma due to some facial distortions. This can result in low self-esteem of the eyeglass wearer and lead to difficulty in making friends and developing relationships.

People with very high-power corrective lenses can benefit socially from contact lenses because these distortions are minimized and their facial appearance to others is normal. Aspheric/atoric eyeglass design can also reduce minification and magnification of the eye for observers at some angles.

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