How Your Eyes Work
How Your Vision is Measured
Your Vision Prescription
Correcting Your Vision

How Your Eyes Work

The eye is one of the most remarkable parts of the human body. 

Light enters your eye through the cornea which is the transparent layer where a contact lens rests when inserted. The cornea helps focus light as it enters the eye, allowing it to pass through the round pupil at the center of the iris -- the colored part of the eye.

The pupil regulates the amount of light entering your eye by constricting in bright light and enlarging in dim light, much like the aperture of a camera. 

Immediately behind the pupil is the retina. The retina focuses the image based on how far the object is from the eye. The sensitive photoreceptor cells of the retina translate the light into electrical impulses and sends them to the brain via the optic nerve.

How Your Vision is Measured

Your vision can be measured in a variety of ways, and at Duke, we use several different methods of measuring vision in order to accurately determine your prescription.

The most common measure of central vision (reading vision) is called Snellen visual acuity. The standard way of determining visual acuity is to have you read letters of decreasing size on an eye chart at a distance of 20 feet. The smallest letters you can read with proper optical correction corresponds to your best-corrected acuity.

Visual acuity is usually expressed as a fraction, with 20/20 being the accepted representation of normal acuity although many of our vision correction patients achieve acuity measurements that surpass the 20/20 level. When acuity falls below this level, the second number in the fraction becomes larger than 20 (for example, 20/30, 20/80, and so on).  

Your Vision Prescription

The diopter is the measuring unit used when describing refractive problems of the eye. Diopters represent the amount of optical correction needed to achieve a person’s best vision. The further from zero your prescription in diopters, the more nearsighted (a negative number) or farsighted (a positive number) you are. 

Nearsightedness, farsightedness, and astigmatism are frequently corrected by glasses or contact lenses. However, there are several procedures available that may reduce or eliminate dependence on corrective lenses.

Correcting Your Vision

Vision correction surgery attempts to significantly improve a person’s uncorrected vision. In many cases, it is possible to achieve unaided 20/20 or better vision, particularly with the advent of customized surgery.  

Vision correction surgery builds on the fact that the cornea plays an important role in focusing the light that ultimately lands on the retina. In the same way that a contact lens helps correct the focusing of light entering into the eye, vision correction surgery changes the shape of the cornea so that its new focusing power matches that of a properly fit contact lens.

Is It the Right Choice for You?

An extensive amount of specialized testing allows your highly trained surgeon to make sure that you are an appropriate candidate for one or more of the various procedures available at the Duke Center for Vision Correction. We strive to offer the best procedure for each individual patient and to deliver care in the safest and most effective manner.

Importantly, we make sure that you do not have an eye disease or systemic condition that might interfere with your surgical procedure, your postoperative healing, or the long term health of your eye.

This is accomplished by a thorough examination and interview with your surgeon before you undergo surgery. Dry eyes, cataracts, glaucoma, diabetes, pupil concerns, and corneal anatomy are all properly screened for and addressed before you are considered an appropriate candidate for vision correction. Astigmatism is measured by six to eight different mechanisms before treatment parameters are prepared.

Results of Vision Correction Surgery

The vast majority of people with low to moderate prescriptions have remarkably predictable results with a single procedure.

Some patients however, have prescriptions that represent extremely high degrees of myopia, hyperopia, or astigmatism, and these patients may not achieve this level of vision with a single procedure. It is possible that those individuals may need spectacles or additional surgery called an enhancement procedure if the target is not achieved in the first procedure.

Our enhancement rate is under 2 percent. It is important to realize that this additional surgery does not represent a treatment error from the first procedure but, rather, a combination of factors involved in the healing process. Therefore, it is important that your expectations for vision correction surgery remain realistic and that you understand that your doctors at Duke are committed to giving you the best possible outcome.