Difference between revisions of Severe myopia

Severe myopia (view source)

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Gene Onco

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 ===Pick lenses with a high index of diffraction===
-The higher the index of diffraction, the smaller the difference in front-back curvature needs to be for a lens to reach a given minus strength. And hence the lower the volume of the glass in the lenses and the weight. Unfortunately, such high-**n** lenses are expensive: Zeiss goes up to **n**=1.9, but these are much more costly than lower **n** glasses. Don't skimp on the anti reflection coatings: the higher the index of diffraction, stronger the specular reflection without coating.
+The higher the index of diffraction, the smaller the difference in front-back curvature needs to be for a lens to reach a given minus strength. And hence the lower the volume of the glass in the lenses and the weight. Unfortunately, such high-''n'' lenses are expensive: Zeiss goes up to ''n''=1.9, but these are much more costly than lower ''n'' glasses. Don't skimp on the anti reflection coatings: the higher the index of diffraction, stronger the specular reflection without coating.
-When you found a frame with very small diameter lenses, going for **n**=1.7 or **n**=1.8 might be good enough to keep the weight down and the edges thin. But the large the diameter, the more necessary it becomes to keep things under control with high-index glass.
+When you found a frame with very small diameter lenses, going for ''n''=1.7 or ''n''=1.8 might be good enough to keep the weight down and the edges thin. But the large the diameter, the more necessary it becomes to keep things under control with high-index glass.
 Beware, high index glass is achieved by admixing lead. Hence the glass is denser and the weight savings somewhat less than the decrease in volume would suggest. Moreover, chromatic aberrations are markedly worse with highly leaded glass[https://en.wikipedia.org/wiki/Chromatic_aberration]. This becomes particularly noticeable when looking off to the side through the lenses, close to their edges.