Difference between revisions of Severe myopia
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Note though that small-diameter lenses clip your field of view. | Note though that small-diameter lenses clip your field of view. | ||
===Opt for lenses with a high refractive index=== | ===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 high-''n'' glass therefore reduces the thickness of the lens edges and thereby significantly 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]: leaded glass weighs more per unit volume. | 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 therefore reduces the thickness of the lens edges and thereby significantly 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]: leaded glass weighs more per unit volume. | ||
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* [https://en.wikipedia.org/wiki/Chromatic_aberration Chromatic aberration] is worse since high-index glass or plastic 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. | * [https://en.wikipedia.org/wiki/Chromatic_aberration Chromatic aberration] is worse since high-index glass or plastic 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. | 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. | ||
===For a large field of view, choose plastic lenses and thick rims=== | ===For a large field of view, choose plastic lenses and thick rims=== | ||
The density of plastic lenses is a few times lower than that of glass lenses. Hence, high-strength plastic lenses with a regular diameter and thus good field of view can still be reasonably light. But the refractive index of plastic lenses goes up to only 1.6 or 1.7 so that plastic lenses will have thicker edges than can be achieved with glass. These thick edges can be masked by choosing a frame with thick rims.<ref name="Jan">https://community.endmyopia.org/t/high-myope-says-hi/18100/42</ref> | The density of plastic lenses is a few times lower than that of glass lenses. Hence, high-strength plastic lenses with a regular diameter and thus good field of view can still be reasonably light. But the refractive index of plastic lenses goes up to only 1.6 or 1.7 so that plastic lenses will have thicker edges than can be achieved with glass. These thick edges can be masked by choosing a frame with thick rims.<ref name="Jan">https://community.endmyopia.org/t/high-myope-says-hi/18100/42</ref> | ||
===Pick frames with a well-matching pupillary distance=== | ===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 when looking straight ahead. 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=== | ===Prefer round or oval frames=== | ||
With the optical axes of your eyes nicely centered in a frame, round frames result in a constant thickness of the lens edge since all of the edge is at the same distance from the optical axis. This means that round lenses minimize thickness and hence weight for a given field of view. | With the optical axes of your eyes nicely centered in a frame, round frames result in a constant thickness of the lens edge since all of the edge is at the same distance from the optical axis. This means that round lenses minimize thickness and hence weight for a given field of view. | ||
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Strong minus glasses significantly compress objects in your field of view such that they appear smaller. The closer the lenses are to your eyeballs, the less compression happens. At the same time, the angular coverage of the lens increases so that your have a larger field of view. Try it: move your minus glasses away from and back towards your eyeballs, and the view compression effect will be apparent. | Strong minus glasses significantly compress objects in your field of view such that they appear smaller. The closer the lenses are to your eyeballs, the less compression happens. At the same time, the angular coverage of the lens increases so that your have a larger field of view. Try it: move your minus glasses away from and back towards your eyeballs, and the view compression effect will be apparent. | ||
When the lenses sit on your eyeballs, view compression is absent and the field of view is not clipped. This is the case when you wear contact lenses. For glasses you can get some | When the lenses sit on your eyeballs, view compression is absent and the field of view is not clipped. This is the case when you wear contact lenses. For glasses you can get these benefits to some degree by choosing a frame with really small diameter lenses and adjusting the nose pads and ear hooks to position the lenses closer to your eyeballs than you would be able to with larger diameter frames: these won't fit between your eyebrow ridge and cheek bones. The practical limit on how deep you can place small-diameter lenses is reached when your eye lashes brush against the lenses. | ||
As a severe myope, you probably learned that pushing your glasses up your nose a bit can bring distant objects into better focus. This is because moving a minus lens closer to your eyeball increases its effective strength. Read the page on [[vertex distance]] to learn the details. For high-strength minus lenses, this results in a significant vertex distance correction that you must take into account when doing the End Myopia program. The vertex distance can change markedly depending on the design and adjustment of your frames. | As a severe myope, you probably learned that pushing your glasses up your nose a bit can bring distant objects into better focus. This is because moving a minus lens closer to your eyeball increases its effective strength. Read the page on [[vertex distance]] to learn the details. For high-strength minus lenses, this results in a significant vertex distance correction that you must take into account when doing the End Myopia program. The vertex distance can change markedly depending on the design and adjustment of your frames. | ||
===Simplifying accounting for vertex distance=== | ===Simplifying accounting for vertex distance=== | ||
By picking the same frame when buying new glasses, the vertex distance will not change. This makes it easier to compare the strengths of successive pairs of glasses. When the frame comes in different colors, you can cycle through the colors to prevent confusing the old glasses for the new ones.<ref name="Jan"/> | By picking the same frame when buying new glasses, the vertex distance will not change. This makes it easier to compare the strengths of successive pairs of glasses. When the frame comes in different colors, you can cycle through the colors to prevent confusing the old glasses for the new ones.<ref name="Jan"/> | ||