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 measure of a visual field in about three minutes. Looking into the automated perimeter, you signal when you see flashes of light. The computer maps your field of vision based on which flashes you see and which you miss.
The next part of a routine eye exam is an external exam, which is a visual inspection of the parts of the eyes that can be seen with just a flashlight. An external exam can be performed quickly. The eye doctor observes the condition of the eyelashes; the position, motions, and skin condition of the eyelids; the actions of the eye muscles (assessed by watching the movements of the eyes); the appearance of the whites of the eyes and the conjunctiva; and the size of the pupils and their reactions, particularly to light.
To see the internal structures of your eyes, the doctor will next ask you to rest your chin on a chinrest and press your forehead against a strap, while he aims an instrument at you called a slit lamp. The slit lamp is both a high-powered microscope and a light source that is focused to form a flat sheet. Because the front parts of the eye are fairly transparent, the sheet of light can show a cross section of the front structures of the eye, the way a sunbeam shining across a room can show the dust in the air. Depending on the width of the light beam and the lens, the slit lamp can give a magnified, three-dimensional view of the cornea, the iris, or the lens, or it can show a cross section from front to back of the eye, through the cornea, aqueous humor, lens, and vitreous. With an additional lens (either a handheld lens or one that fits directly against the cornea), the doctor can see all the way to the retina, blood vessels, and optic nerve at the back of the eye.