Difference between revisions of Severe myopia

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===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 [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 high-''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.

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 reflections at the glass/air interfaces. You really need anti-reflection coatings. But these don't reduce reflections as much as they do for regular glass.

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

* 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.

~~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 lens diameter—and of course the stronger the lenses—the more necessary it becomes to keep the weight down and edge thickness under control by paying for the highest-''n'' lenses you can get.

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, 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===

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 ===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 [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 high-''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.
 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 reflections at the glass/air interfaces. You really need anti-reflection coatings. But these don't reduce reflections as much as they do for regular glass.
-* High refractive index glass is subject to much stronger specular reflections at the glass/air interface.
+* 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.
- 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 lens diameter—and of course the stronger the lenses—the more necessary it becomes to keep the weight down and edge thickness under control by paying for the highest-''n'' lenses you can get.
-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, 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===