Two lenses working together
The eye focuses light with two parts. The cornea, the clear dome at the front, does most of the work. The natural lens, behind the pupil, fine-tunes the focus: small muscles around it change its shape so you can switch between far and near.
When everything is in proportion, light from a distant object lands in sharp focus on the retina at the back of the eye, with no effort. Doctors call this emmetropia. When the length of the eye and the curve of the cornea do not match, the focus falls in the wrong place. That is a refractive error, and it is what glasses, contact lenses and laser correction fix.
Nearsightedness (myopia)
The eye is a little too long, or the cornea too steeply curved. Light from far away comes into focus in front of the retina, so distant things blur. Near things stay clear, sometimes only very close up.
Signs: blurred distance vision, often worse at night; squinting to see far; clear vision up close.
Nearsightedness often runs in families and tends to start in childhood. High degrees come with a higher risk of some other eye conditions, such as retinal detachment, open-angle glaucoma and changes at the centre of the retina, so regular check-ups matter.
Farsightedness (hyperopia)
The eye is a little too short for its focusing power. Light focuses behind the retina. Younger people can often make up for it by focusing harder, so they may see clearly, but with effort. That effort gets harder with age.
Signs: blur at near and sometimes at distance; eyes that tire, or headaches, after reading; needing reading glasses earlier than friends.
Farsighted eyes tend to be small, with a narrow drainage angle, which raises the risk of angle-closure glaucoma.
Astigmatism
A cornea with no astigmatism is curved evenly, like a football. With astigmatism it is curved more in one direction than the other, more like a rugby ball. Light focuses at two different points, so the image is smudged at every distance. Astigmatism often comes together with nearsightedness or farsightedness.
Signs: fuzzy or shadowed edges; mixing up letters with straight lines; letters or lines running together; headaches and glare.
How the vision chart works
On the chart, 20/20 means you can read at 20 feet what a typical eye reads at 20 feet. 20/200 means you can read at 20 feet only what a typical eye reads from 200 feet away. The bottom number is the one that grows as vision gets worse.
Ways to correct it
| Nearsightedness | Farsightedness | Astigmatism | |
|---|---|---|---|
| Glasses | Minus (−) lenses | Plus (+) lenses | Cylinder lenses at a set angle |
| Contact lenses | Yes | Yes | Toric soft or rigid lenses |
| Laser correction | SMILE Pro, LASIK, PRK | LASIK, PRK | SMILE Pro, LASIK, PRK |
| Lens surgery | Implant replaces the natural lens | Implant replaces the natural lens | Toric implant |
Laser correction reshapes the cornea so it focuses correctly. Whether it suits you depends on your prescription, the thickness and shape of your cornea, and your eye health, which a refractive screening measures. Read on: How laser vision correction works.
This guide explains; it does not diagnose. Your doctor’s advice about your own eyes comes first.