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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 "to take", recipere. 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" sounds 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.

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.

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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