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This is an eyeglass prescription.

This article describes the optics of an ordinary eyeglass prescription, which is used to correct small refractive errors in the optical system of the eye. The effect of these errors is to create a blurred image. By correcting these errors, the eyeglass lens removes the blur.

Abbreviations

• R_x means prescription and is derived from the abbreviation "rx" from the Latin word recipire, "to take." Literally, Rx is an instruction to the patient "to take" what is specified in the prescription. See Medical prescription.

• O.D. is an abbreviation for oculus dexter, Latin for "right eye." (Some eyeglass prescriptions simply say "left" and "right" instead of "O.S." and "O.D.") Oculus means "eye." An eye doctor is sometimes called an "oculist" (although eye doctors themselves usually prefer to use either the term ophthalmogist or optometrist). The Latin word dexter means "right" (as opposed to "left"). The words "dexterity" and "dextrous" derive from this root, because the right hand is usually more skillful than the left.

• O.S. is an abbreviation for oculus sinister, Latin for "left eye." The Latin word "sinister" means "left," which was once associated with the idea of evil. The "bar sinister" in a medieval heraldry meant that the bearer was of illegitimate descent.

Note: The word ophthalmologist has an "h" after the "p". The first syllable is pronounced "aahf," like the "oph" in "sophomore." It comes from the Greek root "ophthalmos-" meaning "eye;" ophthalmology is the science of eyes. The word optometrist comes from a different Greek root, "opto-," meaning "vision." An optometrist means literally one who measures vision. In the United States, these are specific terms referring to specific state licensing and courses of professional study. There is considerable overlap in the scope of the two specialties; both ophthalmologists and optometrists are eye doctors with extensive training. Broadly, ophthalmogists tend to lean toward the treatment of eye disease, and optometrists toward the maintenance of optimum vision in healthy eyes.

• D.V. and N.V. stand for "distant vision" and "near vision" and refer to the different corrections in the upper and lower portions of a bifocal lens. The "N.V." portion of a "single vision" lens prescription will be blank.

• Spherical, Cylindrical, and Axis will be explained further below. Most eyeglass prescriptions will contain values here. The "spherical" and "cylindrical" columns contain lens strengths in diopters (see below); "axis" contains the direction of the cylinder axis in degrees.

• Prism and Base are usually blank; they are refer to corrections for errors in eye orientation, such as strabismus.

Blur

Here are some examples of the kind of blurred images that can result from refractive errors; we will be discussing them in more detail below. For now, note that not all kinds of blur are the same.

Lenses

Depending on the optical setup, lenses can act as magnifiers, lenses can introduce blur, and lenses can correct blur. Many people first encounter lenses in the form of magnifying glasses, and think of lenses as magnifiers. Eyeglasses may, in fact, have a small magnifying or reducing effect, but that is an unintentional (and undesirable) side effect. Eyeglasses do not improve vision by magnifying images; they improve vision by reducing blur.

Lens strength

The numbers

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