Difference between revisions of Severe myopia

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

===~~Pick~~ lenses with a high index of diffraction===

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

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.

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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.

===~~Pick~~ 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.

@@ Line 19: / Line 19: @@
 Note though that small diameter lenses clip your field of view.
-===Pick lenses with a high index of diffraction===
+===Opt for lenses with a high index of diffraction===
 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.
@@ Line 30: / Line 30: @@
 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.
-===Pick 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.