Difference between revisions of Severe myopia

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There are disadvantages:

* Cost: Zeiss goes up to ''n''=1.9, but these lenses can cost around $300 each at an optometrist.

* High refractive index glass is subject to much stronger reflections at the glass/air interfaces. You really need [https://en.wikipedia.org/wiki/Anti-reflective_coating anti-reflective coatings], but these do not reduce reflections as much as they do for lower 'n'' glass.

* High refractive index glass is subject to much stronger reflections at the glass/air interfaces. You really need [https://en.wikipedia.org/wiki/Anti-reflective_coating anti-reflective coatings], but these do not reduce reflections as much as they do for lower ''n'' glass.

* [https://en.wikipedia.org/wiki/Chromatic_aberration Chromatic aberration] is worse since leaded glass has significantly more [https://en.wikipedia.org/wiki/Dispersion_(optics) dispersion] than regular glass. This is particularly noticeable when looking off to the side through the lenses, close to their edges, at a multi-color target such as an RGB LED.

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.

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

Lenses are cut to fit the frame such that their optical axis (the thinnest part of a minus lens) sits right in front of the pupil of each of your eyes. If the frame perfectly matches your pupillary distance, the optical axis of the lenses will be right in the middle between the rims of the frame. This minimizes the thickness of the edges, and hence weight.

Lenses are cut to fit the frame such that their optical axis (the thinnest part of a minus lens) sits right in front of the pupil of each of your eyes. If the frame perfectly matches your [[Pupillary Distance|pupillary distance]] , the optical axis of the lenses will be right in the middle between the rims of the frame. This minimizes the thickness of the edges, and hence weight.

===Prefer round or oval frames===

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 There are disadvantages:
 * Cost: Zeiss goes up to ''n''=1.9, but these lenses can cost around $300 each at an optometrist.
-* High refractive index glass is subject to much stronger reflections at the glass/air interfaces. You really need [https://en.wikipedia.org/wiki/Anti-reflective_coating anti-reflective coatings], but these do not reduce reflections as much as they do for lower 'n'' glass.
+* High refractive index glass is subject to much stronger reflections at the glass/air interfaces. You really need [https://en.wikipedia.org/wiki/Anti-reflective_coating anti-reflective coatings], but these do not reduce reflections as much as they do for lower ''n'' glass.
 * [https://en.wikipedia.org/wiki/Chromatic_aberration Chromatic aberration] is worse since leaded glass has significantly more [https://en.wikipedia.org/wiki/Dispersion_(optics) dispersion] than regular glass. This is particularly noticeable when looking off to the side through the lenses, close to their edges, at a multi-color target such as an RGB LED.
 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.
 ===Pick frames with a well-matching pupillary distance===
-Lenses are cut to fit the frame such that their optical axis (the thinnest part of a minus lens) sits right in front of the pupil of each of your eyes. If the frame perfectly matches your pupillary distance, the optical axis of the lenses will be right in the middle between the rims of the frame. This minimizes the thickness of the edges, and hence weight.
+Lenses are cut to fit the frame such that their optical axis (the thinnest part of a minus lens) sits right in front of the pupil of each of your eyes. If the frame perfectly matches your [[Pupillary Distance|pupillary distance]] , the optical axis of the lenses will be right in the middle between the rims of the frame. This minimizes the thickness of the edges, and hence weight.
 ===Prefer round or oval frames===