Refractive Errors
For our eyes to be able to see, light rays must be bent or "refracted" so they can focus on the retina, the nerve layer that lines the back of the eye. The cornea and the lens refract light rays. The retina receives the picture formed by these light rays and sends the image to the brain through the optic nerve. A refractive error means that the shape of your eye doesn't refract the light properly, so that the image you see is blurred.
While refractive errors are called eye disorders, they are not diseases. In a normal eye, the cornea and lens focus light rays on the retina.
A myopic eye is longer than normal, so that the light rays focus in front of the retina. Close objects look clear but distant objects appear blurred. In myopia, distant objects are blurry because the eye is too long, and images focus in front of the retina instead of on it. Myopia is inherited and is often discovered in children when they are eight to twelve years old. During the teenage years, when the body grows rapidly, myopia gets worse. Between the ages of 20 and 40, there is usually little change.
If the myopia is mild, it is called low myopia. Severe myopia is known as high myopia. If you have high myopia, you have a higher risk of detached retina. It is important to have regular eye examinations by an ophthalmologist (medical eye doctor) to watch for any changes in the retina. If the retina does detach, a surgical operation is the only way to repair it.
A hyperopic eye is shorter than normal. Light from close objects, such as the page of a book, cannot focus clearly on the retina. In hyperopia, the eye is too short for images to focus on the retina, so close objects are blurry.
Like nearsightedness, farsightedness is usually inherited. Babies and young children tend to be slightly hyperopic. As the eye grows and becomes longer, hyperopia lessens.
The cornea is the clear front window of the eye. A normal cornea is round and smooth, like a basketball. When you have astigmatism, the cornea curves more in one direction than in the other, like a football. Astigmatism distorts or blurs vision for both near and far objects. It's almost like looking into a funhouse mirror in which you appear too tall, too wide or too thin. You can have astigmatism in combination with myopia or hyperopia.
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.
