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. Lens materials Optical crown glass (B270 - Soda/lime glass) Refractive index (nd): 1.52288 Abbe value (Vd): 58.5 Density: 2.56 g/cm3 (the heaviest corrective lens material in common use, today) UV cutoff: 320 nm Glass lenses have become less common owing to the danger of shattering and their relatively high weight compared to CR-39 plastic lenses. They still remain in use for specialised circumstances, for example in extremely high prescriptions (currently, glass lenses can be manufactured up to a refractive index of 1.9) and in certain occupations where the hard surface of glass offers more protection from sparks or shards of material. If the highest Abbe value is desired, the only choices for common lens optical material are optical crown glass and CR-39. Higher-quality optical-grade glass materials exist (e.g. Borosilicate crown glasses such as BK7 (nd=1.51680 / Vd=64.17 / D=2.51 g/cm3), which is commonly used in telescopes and binoculars, and fluorite crown glasses such as the best optical quality low dispersion glass currently in production, N-FK58 made by the German company Schott with the following characteristics (nd=1.456 / Vd=90.90 / D=3.65 g/cm3) and are commonly used in high-end camera lenses). One must bear in mind that the human eye itself has an Abbe value Vd≈50.2 so the extremely expensive high-end optical glass types mentioned above would be of dubious value when used to make corrective lenses. Also, one would be very hard-pressed to find a laboratory that would be willing to acquire or shape custom eyeglass lenses from these materials, considering that such an order would most likely consist of just two different lenses that are specific to the wearer. In general, Vd values above that of Crown Glass and CR-39 are of dubious value, except in combinations of extreme prescriptions, very large lens sizes that cover a good portion of the face, high wearer sensitivity to dispersion, and occupations that involve work with very high contrast elements (e.g. reading dark print on very bright white paper, construction involving contrast of (dark) building elements against a cloudy white sky, a workplace with recessed can or other concentrated small area lighting shining on very bright white surfaces, etc.). Plastic For CR-39: Refractive index (nd): 1.498 (standard) Abbe value (Vd): 59.3