Eyeglass prescription

An eyeglass prescription is a written order by an optometrist or ophthalmologist to an optician for eyeglasses. It specifies the refractive power to which the eyeglasses are to be made in order to correct blurred vision due to refractive errors, including myopia, hyperopia, astigmatism, and presbyopia. It is typically determined using a phoropter or an automated refractor.

The values indicated in the "sphere" and "cylinder" columns of an eyeglass prescription specify the optical power of the lenses in diopters, abbreviated D. The higher the number of diopters, the more the lens refracts or bends light. A diopter is the reciprocal of the focal length in meters. If a lens has a focal length of 1/3 meters, it is a 3 diopter lens.

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A +10 diopter lens, which has a focal length of 10 centimeters, would make a good magnifying glass. Eyeglass lenses are usually much weaker, because eyeglasses do not work by magnifying; they work by correcting focus. The eye itself has a refractive power of 60 diopters.

Stacking lenses combines their power. A +1 diopter lens combined with a +2 diopter lens forms a +3 diopter system.

Depending on the optical setup, lenses can act as magnifiers, lenses can introduce blur, and lenses can correct blur.

Whatever the setup, spherical lenses act equally in all meridians: they magnify, introduce blur, or correct blur the same amount in every direction.

An ordinary magnifying glass is a kind of spherical lens. In a simple spherical lens, each surface is a portion of a sphere. When a spherical lens acts as a magnifier, it magnifies equally in all meridians. Here, note that the magnified letters are magnified both in height and in width.

The leftmost image above shows a Snellen eye chart as it might be seen by a person who needs no correction, or by a person who is wearing eyeglasses or contacts that properly correct any refractive errors he or she has.

The images labelled 1D, 2D, and 3D give a very rough impression of the degree of blur that might be seen by someone who has one, two, or three diopters of refractive error. For example, a nearsighted person who needs a -2.0 diopter corrective lens will see something like the 2D image when viewing a standard eye chart at the standard 20-foot distance without glasses.

A very rough rule of thumb is that there is a loss of about one line on an eye chart for each additional 0.25 to 0.5 diopters of refractive error.

The top letter on many eye charts represents 20/200 vision. Many people cannot read even this top letter without glasses. The definition of legal blindness is not based on vision without glasses. The U. S. Social Security administration, for example, states that "we consider you to be legally blind if your vision cannot be corrected to better than 20/200 in your better eye." In other words, what matters in evaluating legal blindness, and for many other purposes, is corrected acuity—which line of the chart can be read with proper lenses. One purpose in which uncorrected vision makes a difference is for receiving a driver's licence that is good without lenses; a person who needs lenses may receive one that requires them to wear the said lenses.

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