In the United States, approximately 75% of the adult population uses vision correction, inclusive of both eyeglasses and contact lenses (National Eye Institute). The importance of vision, and in turn corrective lenses for the majority of the population, is indispensable. These lenses allow people to read their favorite books, see the faces of loved ones, look at great works of art—they allow people to see the world.
The Human Eye
To understand how corrective lenses work, there must first be a foundational understanding of how the human eye works. The retina, a layer of cells found on the back of your eye, reacts to incoming light and relays the information to your brain. As scattered light enters your eye, three things must happen to the light for visual clarity:
The size of the image must be reduced to fit onto the retina
The light must focus at the surface of the retina
The curve of the image must be matched to the curvature of the retina.
These three things are accomplished by the eye’s lens and cornea. The lens lies between the retina and the pupil. The cornea is a transparent covering of the lens, as shown in the diagram above.
There are numerous factors that make the eye unable to focus quite right. The lens or corneal surface may not be smooth, the lens may be unable to adjust its curvature to match the image, or the cornea may be misshapen—to name a few.
Often, the root of vision issues result from the eye’s inability to focus the image onto the retina. This is the case for three of the most common vision problems: myopia, hyperopia, and astigmatism. Myopia, commonly known as nearsightedness, is a result of the incoming image being focused in front of the retina. Hyperopia, commonly known as farsightedness, is a result of the incoming image being focused behind the retina. Astigmatism is caused by a distortion (likely from an unsmooth lens and/or corneal surface) that creates as second focal point. This is where corrective lenses come in!
When considering a corrective lens, it is best to think of it as two rounded prisms put together. A prism is thicker at one end, and the light passing through is bent (refracted) toward the thickest portion. When a corrective lens (of the correct type of power) is placed in front of the eye, the focal point will be adjusted to compensate for the eye’s inability to focus the incoming image onto the retina.
Individuals with myopia (nearsightedness) use eyeglasses to see far away. A minus lens is used to correct myopia by pushing the focus farther back. A minus lens is thickest at the base, thus the light is spread away from the lens center. As the strength of the lens increases, the focal point becomes longer.
In the case of hyperopia (farsightedness), eyeglasses are used predominantly to see things up close. Depending on the severity of hyperopia, vision can be blurry for objects at any distance. A plus lens is used to correct for hyperopia by moving the focal point forward. A plus lens is thickest at the center, so it acts to bend light toward the center. As the strength of a plus lens increases, the focal point becomes shorter.
In the case of astigmatism, vision for both near and far objects appears blurry so eyeglasses are used for all distances. A cylindrical lens is used in the case of astigmatism to focus light into a line rather than a point (as a spherical lens would), creating one focal point instead of two.
Corrective lenses are prescribed as both eyeglasses and contact lenses. In both cases, a lens is used to bend the incoming light to compensate for the eye’s imperfections, creating a clear image.