Starting with light and ending in sight
Vision begins at the cornea, the front part of the eye where incoming light is refracted into the pupil, the dark opening in the eye. The iris, the colored ring surrounding the pupil, opens and closes to control the amount of light entering through the pupil. Once past the pupil, the light passes through the lens, which further refracts the light onto the back of the eye.

Transferring visual information to the brain
A thin layer of tissue called the retina covers the back of the eye. The retina is made of millions of light-sensitive photoreceptors and nerves that detect the image formed by the optics of the eye, much like the pixels of a digital camera. These retinal nerves send information to the optic nerve, which transfers information to the brain. If both eyes function properly, information from the eyes is merged into a single, three-dimensional image by the brain, allowing us to determine the distance of objects we see.

Perfectly focused vision
Normal, perfectly focused vision is called emmetropia, derived from a Greek word meaning “equally measured.” The proportions of the eye’s shape, size and structure have to be exact so that light is brought into sharp focus at the back of the eye. The degree to which the lens of the eye focuses light is called refractive power. Your refractive power is measured in units called diopters, abbreviated “D.” Emmetropia measures 0.0 D.

Myopia usually begins to develop between the ages of 6 and 12. The likelihood of this vision defect is increased if one or both parents are nearsighted. Although everyone is different, typically the degree of myopia in children changes gradually every year—sometimes more rapidly—until later in teenage years, when the severity of the refractive error usually peaks. In rare cases, myopia may continue to progress as a person ages.

While myopia cannot be prevented because the eye condition is hereditary or occurs during growth spurts, research is currently focused on slowing the progression of myopia.

Common symptoms of myopia include:

• Headaches from eye strain, for instance after watching TV.
• Difficulty focusing on distant objects, for instance while driving—particularly at night.
  
• Frequent squinting and eye strain when trying to focus on objects in the distance.

Severe myopia can also lead to more degenerative changes in the back part of the eye, such as retinal detachment. That’s why it is important to diagnose and treat myopia. Both ophthalmologists and optometrists can detect the vision defect through a basic eye exam. Myopia is a vision condition, and unless severe, is not a harmful disease. The eye is still healthy but requires visual aids.

Hyperopia usually develops at birth or at a young age, but only causes visual problems in a small percentage of people until the age of 40, when the percentage starts to increase. At a young age, the flexibility of the natural lens can compensate for hyperopia by changing its shape and focusing the light rays exactly on the retina. This ability is known as accommodation.

Around the age of 40, the natural lens can become less flexible, losing its ability to compensate for the hyperopic refractive error. Also, lower degrees of hyperopia become more noticeable and reading glasses may be needed to focus on close objects.

The condition is often inherited, but in rare cases may occur as a result of various diseases.

If your degree of hyperopia is low, you might not notice your compromised vision. However, living without corrected hyperopia could negatively affect your quality of life. Common symptoms of hyperopia include:

• Headaches from eye strain after reading, writing or working on a computer for a long period of time.
• Difficulty concentrating and focusing on objects up close.
• Eye fatigue or burning eyes.
• Irritability or anxiousness while trying to concentrate.
  

It is important to have your eyes examined regularly by an expert. Hyperopia can be diagnosed with a basic eye exam by an ophthalmologist or an optometrist. It is a vision condition, not a harmful disease; the eye is still healthy but requires visual aids.

Astigmatism is not preventable. Like myopia and hyperopia, astigmatism is a refractive error, meaning the eye cannot focus light properly onto the retina. It is usually the result of the cornea not being perfectly round.

Astigmatism may also occur if the eye’s natural lens is not round, a condition called lenticular astigmatism. It can also be a symptom of a disease known as keratoconus, which is recognizable because of a very irregularly shaped cornea.

Astigmatism can be present at birth, during early childhood, or could develop after a trauma injury or disease. If one or both parents have the condition, that also increases the likelihood of astigmatism.

Astigmatism can affect sight in different ways, depending on how the cornea is shaped: Vision may be distorted in a horizontal, vertical and/or diagonal direction. This condition can occur by itself or can be accompanied by or in combination with myopia or hyperopia. Typical symptoms of uncorrected astigmatism can be eye strain and headaches that mostly occur after reading or other prolonged visual tasks, such as working on a computer. A very common symptom is frequent squinting.

Regular astigmatism is a vision condition, not a harmful disease; the eye is still healthy. An ophthalmologist or optometrist can detect astigmatism in a basic eye exam. If you are diagnosed with astigmatism, it is likely you will need glasses, contact lenses or corrective laser eye surgery.

If you are diagnosed with astigmatism, there are several ways your vision may be corrected. You should consult with an ophthalmologist or optometrist to determine which options are available to you.

Glasses are often a convenient way to improve vision. Prescription glasses can fix refractive errors by altering the angle that light rays enter the eye. There is a wide variety of eyeglass frames and lenses to choose from, suiting different styles and budgets.

Contact lenses correct vision the same way as glasses: fixing refractive errors by altering the angle that light rays enter the eye. Contact lenses differ in materials and how long you can wear them. For astigmatism, specially shaped toric contact lenses are available. It is important to note that contact lenses may present potential complications. Be sure to consult with your ophthalmologist or optometrist about any special considerations.

Laser eye surgery is another option to treat astigmatism. If your degree of astigmatism is moderate to high, LASIK or PRK may be recommended by your doctor. These surgeries have high rates of success. Like all surgeries, these procedures are not without potential risks and possible side effects. Your ophthalmologist or optometrist can give you more information.

Intraocular lens exchange is another option your doctor may recommend for correcting severe astigmatism. This is a surgical procedure in which an artificial lens is implanted in place of the eye’s natural lens. Intraocular lens exchange is a common procedure for patients with cataracts; by comparison, refractive or clear lens exchange is for patients without cataracts. Your doctor can advise whether lens exchange is an option for you as well as which IOL is right for your type of refractive error. Like all surgeries, it is not without potential risks. Be sure to get more information from your ophthalmologist or optometrist.

The onset of this condition usually occurs around age 40 but varies from person to person. The decline in your vision might seem sudden, but the loss of flexibility in the natural lens—the cause of presbyopia—progresses over a number of years.

When presbyopia develops and to what degree depends on your individual anatomical eye conditions and any previously existing prescriptions for vision correction or treatment of presbyopia.