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How to Measure Eyeglasses

The question of how to measure eyeglasses probably didn’t occur to people prior to being able to order eyeglasses over the Internet. They went to the eye doctor’s office and picked out a pair of frames after the doctor examined their eyes. Today, with a prescription from the eye doctor in hand, picking out new frames can be done online, eliminating the fitting process.
There are several things to consider before ordering eyeglasses in this way. A visit to the eye doctor is essential. Many tests will be done on the eyes to insure there aren’t any problems beyond the need for a prescription adjustment or a bifocal. Those tests include retinoscopy, when the patient looks at the eye chart with the big “E” and the doctor checks light reflection from the eyes. In another test, refraction, the patient looks through a series of lenses, choosing the ones that make vision clearer. Other tests, using instruments like autorefractors and aberrometers, check how an image focuses on the retina. In a cover test, the doctor covers one eye while the patient looks at a distant object. The doctor checks the eyes for amblyopia and depth perception, among other problems, with the cover test. Other more advanced tests may be given, including a slit lamp exam in which signs of infection or disease, cataracts, corneal ulcers, diabetic eye disorders, and macular degeneration can be discovered. Visual field tests check for blind spots, and dilation is done for more thorough examination of the interior of the eye.
After the tests are performed, the doctor will write a prescription for your eyeglass lenses, and take measurements, like the distance between your pupils. This measurement, abbreviated as PD, is essential to ensuring the optical centers of your lenses will sit directly in front of your pupils. This number is measured in millimeters, and the average distance is about 63 mm. This measurement is extremely important for strong prescriptions. If PD is incorrectly measured, eyestrain and headaches may occur, and vision will not be optimal.

When ordering glasses online, check your old glasses for the numbers you will find either behind the bridge (nose-piece) or on the inside of the arm. The numbers will look like this: 50/17 135 and may be separated by a box shape. These numbers represent the frame size you will need to order. If this is your first pair of glasses, visit an eyeglass store and ask to be fitted, copy the number down, or simply order from the store.

One of the most important frame considerations is whether the patient is nearsighted or farsighted. Lenses for nearsighted people make the eyes appear smaller and because they are concave, are much thinner in the center. Smaller frames not only help the eyes appear larger but are safer, particularly if the lenses are made of glass. Just the opposite is true for farsighted people whose eyes appear too large and whose convex lenses are quite thin on the outside edges.

There are other considerations for choosing frames, though they are of a more aesthetic nature. The shape of the patient’s face - oval, round, square, triangular, oblong, or diamond - is a major determinant in the type of frames that should be chosen. The optical technician or eye doctor can recommend frames that complement the patient’s facial shape. Skin, hair and eye colors should be considered, as well as hairstyle of the patient. For example, a patient with blond hair and blue eyes will look better in lighter frames than a dark-eyed brunette.

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