Difference between revisions of Severe myopia

Line 20:

===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, ~~and~~ 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.

Line 27:

===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 matches your pupillary distance, this will have the optical axis right in the middle of the rims of the frame, which minimizes the thickness of the edges.

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 matches your pupillary distance, this will have the optical axis right in the middle of the rims of the frame, which minimizes the thickness of the edges, and hence weight.

==View compression==

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. Try it: move your glasses away and ~~toward~~ your eyeballs, and the effect will be readily apparent, at least if you are a severe myope.

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. Try it: move your glasses away from and back towards your eyeballs, and the effect will be readily apparent, at least if you are a severe myope.

When the lenses sit on your eyeballs, view compression is absent. This is the case when you wear contact lenses. But for glasses you can get some of this benefit by choosing frames with really small diameter lenses and having the nose pads and ear hooks adjusted to pull the frames closer to your eyeballs ~~there where~~ larger diameter frames ~~would have the lenses touch your face when pulled in that close~~.

When the lenses sit on your eyeballs, view compression is absent. This is the case when you wear contact lenses. But for glasses you can get some of this benefit by choosing frames with really small diameter lenses and having the nose pads and ear hooks adjusted to pull the frames closer to your eyeballs than you would be able to with larger diameter frames.

@@ Line 20: / Line 20: @@
 ===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, and 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.
@@ Line 27: / Line 27: @@
 ===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 matches your pupillary distance, this will have the optical axis right in the middle of the rims of the frame, which minimizes the thickness of the edges.
+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 matches your pupillary distance, this will have the optical axis right in the middle of the rims of the frame, which minimizes the thickness of the edges, and hence weight.
 ==View compression==
-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. Try it: move your glasses away and toward your eyeballs, and the effect will be readily apparent, at least if you are a severe myope.
+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. Try it: move your glasses away from and back towards your eyeballs, and the effect will be readily apparent, at least if you are a severe myope.
-When the lenses sit on your eyeballs, view compression is absent. This is the case when you wear contact lenses. But for glasses you can get some of this benefit by choosing frames with really small diameter lenses and having the nose pads and ear hooks adjusted to pull the frames closer to your eyeballs there where larger diameter frames would have the lenses touch your face when pulled in that close.
+When the lenses sit on your eyeballs, view compression is absent. This is the case when you wear contact lenses. But for glasses you can get some of this benefit by choosing frames with really small diameter lenses and having the nose pads and ear hooks adjusted to pull the frames closer to your eyeballs than you would be able to with larger diameter frames.