How Do Eye Contacts Work?

Concepts

Contact lenses are designed to correct vision problems. There are four basic types of vision problems, and contact lenses work in different ways to correct them. The four basic vision problem types are myopia, astigmatism, hyperopia and presbyopia. These vision problems result in a condition called refractive error, which is the term used when light does not focus properly on the retina, which is at the back of the eye. Eyes can have more than one of these vision problem types at the same time.

Myopia

This is also called near-sightedness. The vision problem that occurs with myopia is that light entering the eye becomes focused too soon, in front of the retina. Contact lenses that correct this problem do so by bending the light as it travels through the contact lenses. It uses a negative power, or concave, lens to spread the light rays slightly farther apart, thereby changing the angle of refraction, so they'll focus on the retina.

Astigmatism

This vision problem occurs when rays of light are focused in different areas of the eye. Contact lenses that correct this problem work by having two refractions in two different directions. This will cause the light rays to focus at a single point. The refractions will be concave or convex depending on whether the person is near-sighted or far-sighted.

Hyperopia

Hyperopia is also called far-sightedness. For people who have far-sightedness, they have eyes with focal points behind the retina. They require contact lenses that bend the light as it travels through them, bringing the rays closer together, which also changes the angle of refraction. This causes a change in the focal point, bringing it closer to the front of the eye, onto the retina. This is called a positive power, or convex, lens.

Presbyopia

This is a condition that is caused by aging. Presbyopia will affect all people as they reach their 40s. It is a condition in which the eye partially loses its ability to focus on objects very close to the eye. This is the reading many people opt for reading glasses to read as they age. Contact lenses that correct for this condition have different areas of the lenses that alter the focal point of light traveling them. It's similar to bifocal lenses in eyeglasses, in which the lower parts act as magnifying lenses.