These are all refractive errors, which means they’re problems with the way the eyes focus light, rather than an eye disease. Refractive errors have to do with the physical shape of our eyes, so let’s take a closer look!
Myopia is the technical term for nearsightedness, meaning that you can see clearly up close but distant objects are blurred. This happens when the eyeball itself is too long, or else when the cornea (the clear front part of the eye) is too curved. That additional curvature or length causes light to focus in front of the retina instead of on it, which makes the resulting images look fuzzy.
The way glasses or contacts correct myopia is by compensating for this error to extend the light’s focus onto the retina where it belongs. These lenses are concave (thinner in the middle), and always have a negative prescription.
Hyperopia, better known as farsightedness, means that you can see distant objects clearly, but everything up close is blurry. Hyperopia happens for the opposite reasons that myopia does. Instead of being too long, the eyeball is too short, or else the cornea is too flat. This causes light to focus behind the retina, making near images fuzzy.
In order to correct hyperopia, corrective lenses must be convex (thicker in the middle) and have a positive prescription. The larger the number, the stronger the prescription.
The third common refractive error people experience is astigmatism, and it’s a little different from the other two. A normal cornea is uniformly curved so that there is a single focal point. A cornea with astigmatism is more football shaped, creating multiple focal points, which makes things appear blurry at any distance and bends their images.
Astigmatism is often paired with one of the other refractive errors, and it requires more complex lenses to correct than they do. Typically, the lens will be somewhat cylindrical rather than spherical.
All three types of refractive error can change and worsen over time, which is why most people who need corrective lenses don’t keep the same prescription forever. If it’s been a while since your last eye exam, or if you’re noticing blurriness where there used to be clarity, having sharp vision again is just one appointment away!