Minus eight diopters? Minus ten? Worse? No doubt you have been on the receiving end of a long series of ever stronger prescriptions from your optometrist or eye doctor. As a severe myope, you probably also spent much time with your nose in books or too close to a screen, landing you on the extreme end of the myopia spectrum.
Basically, your eyeballs are now egg-shaped. Stop being an egg head. Grow some balls. Reverse direction. Do the End Myopia program.
With severe myopia, there are some complications to be mindful of. This page explains them and provides you with guidance.
- 1 Retinal detachment
- 2 Avoid heavy glasses
- 3 View compression and accounting for vertex distance
As your eyeballs elongate with increasing myopia, the retina—which is the light-sensing membrane at the back of the eye—becomes more tensioned. For severe myopes, the risk of retinal detachment is therefore markedly elevated. All the more reason to reverse direction.
Learn to recognize the symptoms: when retinal detachment does happen, fast intervention and treatment can rescue you from losing sight in an eye.
Avoid heavy glasses
The front of minus lenses is flatter than the back, which is curved more strongly. In the center, minus lenses are typically only about 1.5mm thick (thinner would make the lenses too fragile), but moving away from the center (optical axis) the glass becomes thicker. The more minus the diopters, the larger the difference in curvature between the front and back surface of the lens, and the thicker the edge. You can determine how thick the lenses will get if you know all the parameters using this handy thickness calculator.
When choosing a normal frame with regular diameter lenses, you will end up with very thick lens edges and hence uncomfortably heavy glasses that easily slide off your nose and look ugly from the side. What to do?
Choose frames with small-diameter lenses
The smaller the diameter of the lenses once they are cut to fit the frame, the thinner the edges and lower the weight. Such frames are not considered very fashionable, and hence can be hard to find. But with severe myopia, the aesthetic and weight inconvenience of very thick edges make frames with small-diameter lenses very desirable.
Note though that small-diameter lenses clip your field of view.
Opt for lenses with a high refractive index
The higher the 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: leaded glass weighs more per unit volume.
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 lenses are subject to much stronger reflections at the lens/air interfaces. You really need anti-reflective coatings, but these do not reduce reflections as much as they do for lower n lenses.
- Chromatic aberration is worse since high-index glass or plastic has significantly more 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.
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.
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, 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
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.
You probably want a bit more field of view horizontally than vertically, which makes oval frames a good compromise at the cost of thicker edges on the left and right side of the lenses. Avoid rectangular frames since the corners of the rectangles will be relatively far from the optical axis, making the lens edges there unnecessarily thick, ugly, and heavy.
View compression and accounting for vertex distance
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 you 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 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.
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.