Difference between revisions of Severe myopia

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Note though that small diameter lenses clip your field of view.

===Opt for lenses with a high index ~~of diffraction~~===

===Opt for lenses with a high refractive index===

The higher the [https://en.wikipedia.org/wiki/Refractive_index refractive index] (''n''), the smaller the difference in front-back curvature needs to be for a lens to reach a given minus strength. Opting for ''n'' glass reduces the thickness of the lens edges and lowers the volume of the glass in the lens. This reduces weight, but less so than the volume reduction because the high refractive index is achieved by admixing lead in the glass[https://en.wikipedia.org/wiki/Lead_glass]: which makes the glass denser.

The higher the index of diffraction (''n''), 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.

There are disadvantages:

* Cost: Zeiss goes up to ''n''=1.9, but these lenses can cost around 250$ each at an optometrist.

* High refractive index glass is subject to much stronger specular reflections at the glass/air interface.

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 have 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 larger the diameter, the more necessary it becomes to keep weight and edges 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.

Beware, 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.

===Pick frames with a well-matching pupillary distance===

@@ Line 19: / Line 19: @@
 Note though that small diameter lenses clip your field of view.
-===Opt for lenses with a high index of diffraction===
+===Opt for lenses with a high refractive index===
+The higher the [https://en.wikipedia.org/wiki/Refractive_index refractive index] (''n''), the smaller the difference in front-back curvature needs to be for a lens to reach a given minus strength. Opting for ''n'' glass reduces the thickness of the lens edges and lowers the volume of the glass in the lens. This reduces weight, but less so than the volume reduction because the high refractive index is achieved by admixing lead in the glass[https://en.wikipedia.org/wiki/Lead_glass]: which makes the glass denser.
-The higher the index of diffraction (''n''), 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.
+There are disadvantages:
+* Cost: Zeiss goes up to ''n''=1.9, but these lenses can cost around 250$ each at an optometrist.
+* High refractive index glass is subject to much stronger specular reflections at the glass/air interface.
+ 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 have 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 larger the diameter, the more necessary it becomes to keep weight and edges 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.
+Beware, 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.
 ===Pick frames with a well-matching pupillary distance===