 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.