Vision Defects and Corrections

When the eye does not focus on the retina

For clear vision, the cornea and lens refract incoming light so that its rays meet on the retina, the light-sensitive layer at the back of the eye. If the rays meet elsewhere, the image on the retina is blurred. Changing the shape of the eye’s lens during accommodation normally helps it focus on near or distant objects, but it cannot always bring the image onto the retina. Two common defects are myopia and hyperopia.

Myopia: distant objects look blurred

A person with myopia, or short-sightedness, can usually see nearby objects more clearly than distant ones. Light from a distant object comes to a focus in front of the retina. This can happen if the eyeball is too long or the eye refracts light too strongly. By the time the rays reach the retina, they have spread out again.

A concave spectacle lens, thinner in the middle than at the edges, refracts the incoming rays so that they spread out slightly before entering the eye. The eye then brings them together farther back, on the retina. This is why myopia needs a diverging lens rather than a lens that makes the rays meet even sooner.

Hyperopia: nearby objects look blurred

A person with hyperopia, or long-sightedness, has difficulty focusing on nearby objects. Their light rays have not yet met when they reach the retina: if continued, the rays would meet behind it. The eyeball may be too short, or the eye may not refract the light strongly enough.

A convex spectacle lens, thicker in the middle, makes the incoming rays converge more before they enter the eye. Together with the eye’s own refraction, this brings the focus forwards onto the retina.

Reading a ray diagram

Follow the arrows to see which way the light travels. First find the retina at the back of the eye, then find where the rays meet. A meeting point in front of it shows myopia; rays that would meet beyond it show hyperopia. In a corrected diagram, look for the spectacle lens in front of the eye and check that the rays now meet on the retina.

Other ways of correcting vision

Spectacles are the usual treatment in this specification, but they are not the only option. Hard or soft contact lenses sit on the surface of the eye and refract light to correct its focus. Laser surgery changes the shape of the cornea, altering how it refracts light. Another option is surgery to replace the eye’s lens with an artificial one. These methods change where the rays focus; the aim is still a sharp image on the retina.