Near- and Farsightedness, Astigmatism and Presbyopia

What are refractive errors?
Clear vision depends on this: the cornea (the clear front window of the eye) and the lens (located behind the pupil) must properly focus light rays on the retina. The retina lines the back wall of the eye. It receives the picture formed by the light rays and sends the image to the brain through the optic nerve. This process produces the sense of sight.

This is very similar to the way a camera takes a picture. The cornea and lens in your eye act as the camera lens. The retina is similar to the film. If the image is not focused properly, the retina or film receives a blurry image. This condition in the human eye is known as a refractive error, because light rays are not refracting — or focusing — properly.

What causes refractive errors?
Differences in the length or shape of the eye can result in blurry vision. There are four types of refractive errors:

Myopia (Nearsightedness)
People with myopia can see close objects, but are unable to see distant objects clearly. If your eye is too long, or your cornea has too much focusing power, images focus in front of the retina. Myopia affects more than 25% of all Americans. It is inherited, and usually detected during childhood. Typically, the condition worsens as the body grows during teenage years and levels off in adulthood.

Hyperopia (Farsightedness)
Hyperopia or farsightedness is the opposite of myopia. Distant objects are clear, and close-up objects appear blurry. This condition is a result of an eye that is too short or a cornea that lacks the necessary refractive power to focus images on the retina. If you have hyperopia, images focus on a point beyond the retina. This unfocused image captured by the retina is then sent to the brain and processed as an unclear picture.

Usually inherited, hyperopia affects about one-fourth of Americans and the incidence increases with age. About half of all people over 65 have the condition.

Astigmatism (Distorted vision)
Astigmatism causes blurry vision when looking at objects that are close, such as reading a letter, and objects that are far away, such as reading a road sign. Vision with astigmatism may not be equally blurry; some distances are more out of focus than others.

A normal cornea is round with even curves from side to side and top to bottom. If you have astigmatism, your cornea is shaped more like the back of a spoon, curved more in one direction than in another. Light rays have more than one focal point and focus on different areas of the retina.

Presbyopia (Aging eyes)
When you are young, the lens in your eye is soft and flexible. The lens of the eye changes its shape easily, allowing you to focus on objects both close and far away. After the age of 40, the lens becomes more rigid. Because the lens can't change shape as easily as it once did, it is more difficult to read at close range. This perfectly normal condition is called presbyopia. You can also have presbyopia in combination with myopia, hyperopia or astigmatism.

How are refractive errors corrected?
Eyeglasses and contact lenses are effective in correcting all of these conditions. Today, laser surgery is also an option for treating myopia, hyperopia and astigmatism. Click here to learn more about LASIK and how it might work for you.

Consult an Eye Care Associates doctor
To learn more about refractive errors — and how to treat them — make an appointment with an experienced eye doctor at Eye Care Associates of Greater Cincinnati.