Editing Corrective lens (section)

=== Optical crown glass ===
* [[Refractive index]] ({{mvar|n}}{{sub|d}}): 1.52288
* [[Abbe number]] ({{mvar|V}}{{sub|d}}): [https://www.knightoptical.com/_public/documents/1372681801_sheetglassb270tsgb270.pdf 58.5]{{Dead link|date=September 2023 |bot=InternetArchiveBot |fix-attempted=yes }}
* [[Density]]: 2.56 g/cm³ (the heaviest corrective lens material in common use, today)
* [[Cutoff (physics)|UV cutoff]]: [https://www.knightoptical.com/_public/documents/1372681801_sheetglassb270tsgb270.pdf 320 nm]{{Dead link|date=September 2023 |bot=InternetArchiveBot |fix-attempted=yes }}

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 number 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 glass|Borosilicate crown glasses]] such as [http://www.pgo-online.com/intl/katalog/BK7.html BK7] {{nobr|( {{mvar|n}}{{sub|d}} {{=}} 1.51680 ,}} {{nobr| {{mvar|V}}{{sub|d}} {{=}} 64.17 ,}} {{nobr| {{mvar|D}} {{=}} 2.51 g/cm³ ),}} which is commonly used in telescopes and binoculars, and [[fluorite]] [[crown glass (optics)|crown glasses]] such as the best optical quality low dispersion glass currently in production, [https://www.schott.com/advanced_optics/us/abbe_datasheets/schott-datasheet-n-fk58.pdf N-FK58] made by the German company Schott with the following characteristics {{nobr|( {{mvar|n}}{{sub|d}} {{=}} 1.456 ,}} {{nobr| {{mvar|V}}{{sub|d}} {{=}} 90.90 ,}} {{nobr| {{mvar|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 [https://application.wiley-vch.de/books/sample/3527410686_c02.pdf Abbe number {{nobr| {{mvar|V}}{{sub|d}} ≈ 50.2 }} ] so the expensive, high-end optical glass types mentioned above have little value for central vision; however, the wearer's view through the side of a glass lens is not comparable to central vision through the eye: Low dispersion glass definitely makes optically superior corrective lenses since it greatly reduces color-fringing of edge-wise viewed contrasty objects, compared to all available plastics. But glass lenses are much heavier, and making them requires specialized glass grinding equipment no longer common in ordinary prescription lens labs: At present, one is often hard-pressed to find an optical laboratory that has the machinery needed shape custom glass lenses.

A further complication for seeking lenses made of better, even-lower dispersion glass, is that the specialty glass is often expensive. Also, many exotic glass types, with [[Abbe number]] {{nobr|{{mvar|V}}{{sub|d}} {{math|≳}} 65 ,}} contain oxides of heavy metals such as [[arsenic]] or [[lanthanum]], some of which are toxic. The need for special venting to protect technicians from exposure to powdered toxic glass further limits the number of optical labs that can safely grind super-low dispersion / large Abbe number glass.

Abbe numbers ({{mvar|V}}{{sub|d}}) in excess of Crown Glass and CR-39 are mainly warranted only for unusual special uses, such as
* extreme positive or negative [[diopter]] prescriptions
* very large size lenses, such as might cover a good portion of the face
* low wearer tolerance of color fringing
* occupations that involve work with very high contrast elements (e.g. reading dark print on white paper under bright light)
* construction work that requires viewing contrasting dark building elements against a cloudy white sky
* workplace with bright recessed can lighting or other darkened room lighting, concentrated on a small areas, shining on bright reflective surfaces (e.g. display counters at jeweler stores).