How Your Eye Works

The "white part" of  the eye is known as the sclera. 

Light enters your eye through the cornea, which is made of transparent tissue. The cornea focuses and bends the light, allowing it to pass through the pupil. 

The pupil is the dark, round opening in the center of the colored iris.  It acts as an aperture to regulate the amount of light entering your eye. The pupil becomes smaller in bright light and larger in dim light.

Directly behind the pupil is the lens. Muscles in the eye adjust the lens to focus the light rays on to the retina, especially during the reading process.

The retina is made of photosensitive cells. It captures light images and translates them into electrical impulses which travel to the brain via the optic nerve.

There is a clear "gel-like" substance, vitreous, inside the eye's cavity which helps maintain the spherical shape of the eye.

The Causes of Blurry Vision

The contour of your cornea, the power of your lens, and the length of your eye determine how clearly you can see. If all three of these elements are matched together perfectly, light rays are properly focused and you can see clearly.

If even one of these elements does not coordinate with the others, light will not focus correctly. These "refractive" or focusing problems are referred to as myopia, hyperopia, astigmatism, and presbyopia.

The measuring unit used when describing refractive problems of the eye is the diopter. Diopters express the focusing power of the corrective lenses that are needed to make the vision normal. The higher your prescription in diopters, the more nearsighted or farsighted you are.

Visual acuity often changes over time, usually declining with age. Certain illnesses, such as diabetes, can also cause changes in vision. Therefore, general health plays a role in how well you see, and should be monitored regularly by a family physician.

Myopia

Otherwise known as nearsightedness, myopia is the most common refractive vision problem. People with myopia can see near objects more clearly but have difficulty focusing on distant objects.

Nearsightedness is either caused by a cornea that has more curvature than normal or an eye that is too long. These conditions cause the light rays to focus at a point in front of the retina

Hyperopia

Hyperopia is more commonly referred to as farsightedness and is the opposite of myopia. Farsighted people have difficulty focusing on nearby objects.

Farsightedness is either caused by a cornea that is flatter than normal or an eyeball that is too short. These conditions cause the light rays to focus at a point behind the retina.

Astigmatism

Astigmatism often occurs along with myopia or hyperopia. This condition is caused by a cornea that has an uneven or asymmetrical curvature. This prevents the eye from properly focusing at any distance and causes a ghosting or doubling effect.

Presbyopia

The normal aging process causes the eye's natural lens to lose some of the flexibility characterized by youth. This condition, along with progressive weakening of the focusing muscles with age, is called presbyopia. Sooner or later, everyone experiences presbyopia, but it usually begins to occur between 40 and 50 years of age. This is why nearsighted people begin to wear bifocals in their 40's, and people who never needed corrective lenses before begin using reading glasses.

Anyone considering refractive surgery should be aware of the impact of presbyopia. Even after a person's distance vision has been corrected to normal, he or she can expect presbyopia to develop with age. At some point, reading glasses will be required for clear sight up close.

Monovision

For patients who would like to delay the use of reading glasses, there is an option known as monovision. This process has been used for years in the fitting of contact lenses. The most common technique is to correct the dominant eye for distance vision and the leave the non-dominant eye somewhat nearsighted so that it can be used for reading. The amount of nearsightedness left can be varied for different purposes. The disadvantage of leaving significant amounts of nearsightedness is that the reading eye will not have good distance vision. This may cause a decrease in depth perception, which is based on the input of sharp visual information to the brain from both eyes.

Some people readily adapt to this condition, while others never become completely comfortable using each eye for different tasks. If a person previously achieved successful monovision with contacts, he or she should have good results with surgical correction as well. Even monovision cannot permanently eliminate the need for reading glasses, but it can definitely increase the length of time that a person can function without corrective lenses.

FDA trials are currently underway in an attempt to surgically improve our ability to focus after forty. It will be a number of years before the results of these early tests are known.