How to Read Your Eyeglass Prescription
When you look at your prescription for eyeglasses, you will see numbers listed under the headings of OS and OD. They are Latin abbreviations; OS (oculus sinister) means the left eye and OD (oculus dextrus) means the right eye. Occasionally you will see a notation for OU, which means something involving both eyes. In general, the further away from zero the number on your prescription, the worse your eyesight and the more vision correction you need. A plus sign in front of the number means you are farsighted and a minus sign means you are nearsighted. These numbers represent diopters, the unit used to measure the correction, or focusing power, of the lens your eye requires. Diopter is often abbreviated "D."
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It is common for pharmacies, supermarkets, airports, and even dollar stores today to carry nonprescription eyeglasses,
sunglasses, and magnifiers. We hope this review will assist consumers with valuable information about these eye products.
Nonprescription eyeglasses are primarily for people who can no longer read fine print. This condition, called presbyopia or "old vision," usually begins at about the age of 35 and progresses until about the age of 55. This inability to focus clearly at near distances is a universal condition associated with human aging. Presbyopia is not a disease, nor does it represent a degenerative process or presage blindness. Presbyopia is simplistically a result of gradual hardening of the eye's natural lens proteins, rendering the lens less elastic and thus less able to change its shape and less able to alter its focal length to nearer objects.
Although over-the-counter (OTC) glasses do not appear to harm vision, if they are not appropriate for a person's eyes, they can cause eyestrain, or asthenopia, after prolonged use. In addition, OTC glasses have the same focusing power in both the left and the right eye, which often does not match the exact prescription of many patients who have a significant difference in the power of their eyes. This power mismatch, called anisometropia, can also cause eyestrain or asthenopia even though the OTC glasses may be correct for one eye.
Most vendors offer standard and bifocal lenses, and some vendor locations also offer transitional lenses that lighten up indoors and darken when outdoors. Reading glasses may be available in both full reading vision or in bifocals. Bifocals, invented by Benjamin Franklin in 1790, have a lower segment to see up close, with an upper segment consisting simply of clear glass for distance vision.
The quality and durability of OTC eyeglasses vary widely. Cheap models -- costing as little as a few dollars -- may have lenses that scratch easily and distort vision. Expensive models -- costing $50 or more -- may incorporate some or all of the desirable characteristics of high-quality prescription lenses. There is no permanent harm to the eyes from wearing poorly focused or distorted lenses. However, eye strain, computer vision problems, and poor quality vision from cheap or improperly fitted eyeglasses can reduce productivity and accuracy.
Manufacturers rate the power of nonprescription glasses from weakest (+1.00 diopters) to strongest (+3.50 diopters or more). When making a selection, the customer tries on eyeglasses and reads fine print while standing 12 to 14 inches from the print, or simply holding the printed material at a comfortable, customary reading distance. The customer keeps trying on glasses with different power ratings until the print comes into sharp focus. Suppose, for example, the print comes into focus when the customer is wearing eyeglasses with a power rating of +2.00. The customer then knows that a +2.00 rating is correct and can select from styles with that rating. This same customer may prefer a lower power, such as a +1.25, for intermediate distance work such as knitting or using a computer. Intermediate work generally focuses at a distance of 16 to 24 inches.
As we age, our eyes continue to gradually lose the ability to focus up close. Thus, a younger person first requiring reading glasses may require only +1.00 diopter lenses, which may progress with time to +2.50 diopter lenses.
Medically Reviewed by a Doctor on 3/24/2016