Familiar vision problems

Over half the population in the United States experiences one or more common visual acuity problems. Generally, people with visual acuity problems are nearsighted, farsighted, and/or have astigmatism. Collectively, these conditions are medically referred to as lower order aberrations. Lower order aberrations are usually corrected by wearing glasses, contact lenses or by laser vision correction.

Understanding your prescription

If you've seen the vision prescription your optometrist or ophthalmologist prepares following an eye exam, you've probably seen a series of positive or negative numbers, one for each eye. These values are known as diopters, and they typically measure the refractive power of your eyes on a scale of -14 to +14.

A person with 20/20 vision will most likely have a reading of zero diopters. A person with myopia (nearsightedness) would have a negative diopter value; a person diagnosed with hyperopia (farsightedness) would have a positive diopter value.

Regardless of whether the number is negative or positive, a higher number indicates a higher refractive error. Your prescription specifies just how much vision correction is required to correct your particular vision problems.

To understand these refractive errors, compare the information and illustrations below of an eye with good visual acuity with a myopic (nearsighted), hyperopic (farsighted) or astigmatic eye.
 

Good visual acuity

What you've probably always heard is true: the human eye does work like a camera. The light and images we see pass through the cornea at the front of the eye. The light and images then go through the lens inside the eye, and, finally, focus directly onto the retina, at the back of the eye. The retina sends the "signals" to our brain, which registers them.

Poor visual acuity is primarily caused by refractive errors. These errors occur when the cornea is shaped in such a way that the images we see do not focus directly on the retina.

Myopia (Nearsightedness)

When you're nearsighted or myopic, images in the distance will seem blurry. It is difficult to see far. Your eyes may be longer than normal or the cornea may be too curved, so images focus in front of the retina.