rnea, a clear, hard tissue covering the iris and the pupil. The curve of the cornea bends, or refracts, light rays, focusing them on the retina at the back of the eye. A pool of fluid called aqueous humor fills a cavity between the cornea and the iris. Directly behind the iris is the lens, an elastic disc about the size and shape of an M&M candy, which flexes to fine-tune focus.
Lining the back of the eyeball is the retina, a complex, photosensitive membrane of many layers. This is the “film” of the eye and its most important part. When light is focused onto the retina, photosensitive cells translate the light into electrical impulses, which are then sent via the optic nerve to the brain, where an image is formed. (See “Anatomy of the Eye” for illustrations.)
The eye exam
Like most doctor visits, the eye exam begins with a little paperwork. You will be asked to answer questions or fill out a form, providing information about your general health, any medicines you take, allergies or eye problems you have, and your family medical history. While some of this information may seem irrelevant, asking these routine questions is the only way to establish background information that really does matter. Having high blood glucose or even taking a common, over-the-counter medicine can cause fluctuations in your vision that might make a difference in your exam.
Background complete, the next step in most eye exams involves assessing your visual acuity, or how well you can see. Vision is measured by the size of the letters you can easily read on the eye chart, which is usually about 20 feet away. If you cannot read all of the letters on the chart, it’s because the shape of your eyeball, lens, or cornea causes light to focus either in front of or behind rather than right on the retina. Using a process called refraction, the eye doctor can find an eyeglass or contact lens prescription that bends the light correctly and enables you to see clearly.
Refraction can be done in several ways. The doctor or a technician may hold up various lenses and ask questions about which combination helps you see best. He may shine a special light into your eyes to measure its shape (a process called retinoscopy), or he may use any one of several instruments that do automated retinoscopy. Each eye is tested separately, then both are tested together. In routine eye exams, if you already wear glasses, your current glasses prescription is read in a machine called a lensometer. The strength of the present prescription is then compared to the best possible correction, determined by refraction.
People sometimes ask why they need to have their glasses and vision checked if they don’t feel they need new glasses. Refraction is routinely performed not necessarily to prescribe new glasses but to determine how well the person can see with the best possible lenses. If a person does not have normal visual acuity even with the optimal correction, it could be a sign of a more serious problem. (In nonroutine eye exams, such as those done by a retinal specialist, refraction is rarely done.)
A person’s vision normally means his central vision, or what he can see looking straight ahead. Everything a person can see up, down, and sideways while looking straight ahead is called peripheral vision. Peripheral vision is measured and recorded as a “visual field.” Measuring the visual field is often part of a routine eye exam. The test can be as simple as noting how far out to the side you can see the doctor’s wiggling pencil while looking straight ahead, or it can be more sophisticated.
In the old days, doctors tested visual field by having a person look at a black felt screen with one eye at a time, while they moved a small circle on a stick from the edge toward the middle of the screen until the person could see it. Sticking a pin in the felt at that spot, they repeated the test from different angles, finally drawing the pattern of pins on a sheet of paper. That method gave reliable information, but it was time-consuming. Now there are automated perimeters that can give an accurate