Severe myopia

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.

Retinal detachment

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[1]. All the more reason to reverse direction.

Learn to recognize the symptoms[2]: 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, and the thicker the edge of the lenses.

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 reduces the thickness of the lens edges and 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[3]: which makes the glass denser.

There are disadvantages:

• Cost: Zeiss goes up to n=1.9, but these lenses can cost around 250$ each at an optometrist.
• High refractive index glass is subject to much stronger reflections at the glass/air interfaces. You really need anti-reflection coatings. But these don't reduce reflections as much as they do for regular glass.
• Chromatic aberrations are markedly worse with highly leaded glass[4]. This becomes particularly noticeable when looking off to the side through the lenses, close to their edges.

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.

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. 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. 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 be adjusted to pull the frames closer to your eyeballs than you would be able to with larger diameter frames as these won't fit between your eyebrow ridge and cheek bones. The practical limit of 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 frames up your nose a bit will 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 have to take into account when doing the End Myopia program.