Eyeglass prescriptions

A refraction is the process of determining an individual’s eyeglass prescription.  A patient generally views an eye chart through the lenses of a phoropter, and a doctor (or well-trained technician) will ask for the patient’s subjective assessment of a series of lens combinations (e.g., “which is clearer, 1 or 2?”).  The doctor will use different types of lenses within the phoropter to adjust for near- or far-sightedness, astigmatism, and for how a patient’s eyes work together.  There are also automated machines available to determine prescriptions, but most doctors use these results as a starting point and then refine the prescription using a phoropter.

Because the refraction may take several minutes and vision deteriorates when a person’s eyes are dry, patients should blink a few times during the refraction to lubricate their eyes.  Patients, and particularly diabetics, should also understand that if their vision at exam time is not representative of their usual vision, the prescription may not be satisfactory.

The doctor’s ultimate goal is to prescribe a lens power that is suited to each patient’s individual vision needs, so there is no ‘correct’ answer during a refraction.   For example, engineers typically prefer crisp, sharp near vision, while avid hunters may place more emphasis on distance vision.   Rather than being an anxiety-filled process, patients should be relaxed and give their honest feedback on the clarity of their vision.

As part of the refraction process, the doctor or technician may tell you how well you see with or without glasses.  For decades many people have referred to perfect vision as “20/20” even without quite understanding what the term means.

The term ‘20/20’ is associated with the Snellen chart, a tool developed in 1862 to measure a person’s visual acuity.  The Snellen chart is often used for school vision screenings, and a version of it is incorporated into the projectors that are used in most doctor offices.  In order to assess a patient’s vision, a patient views the Snellen chart from 20 feet away (or a projector image is manipulated with mirrors so that the chart appears 20 feet away) and reports which letters are clearly legible.

If a person has 20/20 vision, he is able to read at 20 feet what the normal eye can read at 20 feet; if vision is 20/50, then he is able to read at 20 feet what the normal eye could read at 50 feet.  The first number of the ‘20/20’ measurement represents the distance in feet at which the test was measured; the second number compares the patient’s vision to the normal eye.

Some patients may have vision that is even better than the normal eye, and thus may have visual acuity of 20/15 or 20/10.  In addition, visual acuity can be tested with or without correction, so a near-sighted patient who sees 20/400 without glasses may easily see 20/20 with glasses.  Unfortunately, some patients, particularly those with eye disease or cataracts, can’t be corrected to 20/20 no matter how strong their glasses are.