Eyeglass prescription

If an eye examination indicates that corrective lenses are appropriate, the prescriber generally provides the patient with an eyewear prescription at the conclusion of the exam.

The parameters specified on spectacle prescriptions vary, but typically include the patient's name, power of the lenses, any prism to be included, the pupillary distance, expiration date, and the prescriber's signature.

The prescription is typically determined during a refraction, using a phoropter and asking the patient which of two lenses is better, or by an automated refractor, or through the technique of retinoscopy.

Convergent powers are positive (e.g., +4.00 D) and condense light to correct for farsightedness/long-sightedness (hyperopia) or allow the patient to read more comfortably (see presbyopia and binocular vision disorders).

Divergent powers are negative (e.g., −3.75 D) and spread out light to correct for nearsightedness/short-sightedness (myopia).

If neither convergence nor divergence is required in the prescription, "plano" is used to denote a refractive power of zero.

For example, a toric lens, when rotated correctly, could focus an object to the image of a horizontal line at one focal distance while focusing a vertical line to a separate focal distance.

For regular toric lenses, these powers are perpendicular to each other and their location relative to vertical and horizontal are specified by the axis component.

Minus cylinder notation is also more common in Asia, although either style may be encountered there.

When dealing with toric lenses, the axis defines the orientation of the steepest and flattest curvatures relative to horizontal and vertical.