The Human Eye and How Vision Works

Sight is our most precious sense. Our eyes enable us to take in the surrounding world. Without sight, the way we perceive the world would be forever changed. No wonder the eyes are often elevated in literature, art, religion, and philosophy to symbolize everything from the windows of the soul to supreme wisdom. Indeed, the eyes are a marvel of mechanics.

However, changes within the eyeball may occur, resulting in impaired vision. Objects that we once viewed with crystal clarity may become blurred or distorted. To better understand how vision may change, let's first examine the anatomy of the eye.

How the Eye Works

You may have heard the comparison between a camera and the human eye. Just as a camera takes in light and transforms it into an image on film, your eye does virtually the same thing, only the "film" is your retina and your brain "develops" the image. We see objects when light, which is reflected by the objects, passes through the eyeball lens and strikes the retina, at the back of the eye. Our brains then interpret the shapes, colors, and dimensions of the objects we see. A clearly focused object is the result of normal vision. However, just as an improper amount of light entering a camera lens will distort a photo, if light entering the eyeball does not strike the retina, the result may be distorted vision.

Anatomy of the Eye

Sclera and Cornea

The outer layer surrounding the eyeball is made up of two parts: the sclera and the cornea. The sclera — the white, opaque part of the eye — makes up the back five -sixths of the eye's outer layer and provides protection for the eyeball. The cornea, about the size of a dime and as thick as a credit card, makes up the remaining sixth of the eye's outer layer. It is the transparent dome, similar to the crystal of a wrist watch, at the front of the eyeball. The cornea provides most of the eye's focusing power, so small changes in its curvature can make an enormous difference in how clearly you see objects.

The cornea has three main layers. The epithelium is the thin outer protective layer of cells; it is made up of the same kind of tissue that covers most of your body, and is continually regenerating, or renewing itself. The stroma is the strong, fibrous layer that makes up 90 percent of the cornea's thickness and provides the cornea with its structure and shape. The endothelium is the single cell layer that lines the inside of the cornea and helps regulate the cornea's fluid content.

Iris

The iris, which determines one's eye color, is located behind the cornea. It is composed of connective tissue and smooth muscle fibers. The muscles of the iris control how much light passes through to the retina.

Pupil

The pupil appears as a black circle in the middle of the iris. The pupil can be likened to the aperture, or shutter, of a camera. When it is very bright, as on a sunny day, the iris muscles make the pupil constrict, or become small, so only a small amount of light will pass into the eye. In darkness, the opposite happens, and the pupil dilates, or enlarges, to let in more light.

Lens

The lens is a circular structure located directly behind the pupil and held in place by slender, strong ligaments. Although most of the bending of light is accomplished by the cornea, the curved lens fine-tunes the angle of light passing through it, focusing the light onto the retina. When the ligaments tighten, the lens becomes flatter, or less convex, allowing you to see objects at a distance. When the ligaments relax, the elastic lens becomes rounder, or more convex, like a magnifying glass, so you can see objects that are close. This ability of the lens to refine the focus through flexing is called accommodation.

Vitreous Humor

The vitreous humor is the jellylike substance, about 99 percent water, that fills the space between the lens and the retina on the inner back wall of the eye. Light passes through the vitreous humor before striking the retina.

Retina

The retina is a complex layer of nerve tissue that lines the inside back wall of the eyeball. Similar to film in a camera, the retina "captures" the image through an electrochemical reaction to light. Electrical impulses are then transmitted through the optic nerve to the brain, which interprets, or "develops," the image.

Common Vision Problems

Your eye doctor may refer to your vision problem as your refractive error, or focusing problem. How well you see is determined, for the most part, by how accurately your eyes are able to bend, or refract, light. In a normal eye, the focus comes to a point on the retina. But sometimes this does not occur. The result? Various forms of vision impairment, or aberrations. Vision problems fall into one of two basic groups: low-order aberrations and higher-order aberrations.

Low-Order Aberrations

These aberrations include common refractive errors, such as nearsightedness, farsightedness, and astigmatism. The impairments can be corrected with eyeglasses, contact lenses, or LASIK, PRK, or other laser vision correction procedures.

Myopia (Nearsightedness)

Also known as nearsightedness, myopia is a condition in which you can see nearby objects well, but objects at a distance appear blurred. This happens when light bouncing off a faraway image enters the eye through the cornea and comes to a point of focus too soon, before it reaches the retina. Myopia may be due to a cornea that has too much curvature, which causes the light to "overbend" and focus in front of the retina. Myopia also occurs when the eyeball is too long — the retinal wall is too far back for the combined focusing power of the cornea and lens.

Hyperopia (Farsightedness)

People with hyperopia, or farsightedness, see distant objects more clearly than nearby objects when they are young but may have difficulty with both as they get older. In hyperopia, the light rays coming into the cornea are not bent sharply enough and are focused behind, rather than on, the retina. The result is a blurred image. This usually happens in people whose eyeballs are too short from front to back or whose focusing muscles around the lens are too weak. Another cause of hyperopia, though rare, is a cornea that is not curved enough.

Because muscles are more elastic in youth, younger people who are mildly hyperopic can actually compensate for it by using the focusing muscles around the lens to fine-tune the focus by bending light more steeply. This action...