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An eyeglass prescription is an order written by an eyewear prescriber, such as an optometrist or ophthalmologist, that specifies the value of all parameters the prescriber has deemed necessary to construct and/or dispense corrective lenses appropriate for a patient. If an 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 automated refractor, or through the technique of retinoscopy. A dispensing optician will take a prescription written by an optometrist or ophthalmologist and order and/or assemble the frames and lenses to then be dispensed and sold to the patient.

Every corrective lens prescription includes a spherical correction in diopters. Convergent powers are positive (e.g., +4.00 D) and condense light to correct for farsightedness (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 (myopia). If neither convergence nor divergence is required in the prescription, "plano" is used to denote a refractive power of zero.

The term "sphere" comes from the geometry of lenses. Lenses derive their power from curved surfaces. A spherical lens has the same curvature in every direction perpendicular to the optical axis. Spherical lenses are adequate correction when a person has no astigmatism. To correct for astigmatism, the "cylinder" and "axis" components specify how a particular lens is different from a lens composed of purely spherical surfaces.

Patients with astigmatism need a toric lens to see clearly. The geometry of a toric lens focuses light differently in different meridians. A meridian in this case is a plane that is perpendicular to the optical axis. 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.

The power of a toric lens can be specified by describing how the cylinder (the meridian that is most different from the spherical power) differs from the spherical power. Power evenly transitions between the two powers as you move from the meridian with the most convergence to the meridian with the least convergence. 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.

There are two different conventions for indicating the amount of cylinder: "plus cylinder notation" and "minus cylinder notation". In the former, the cylinder power is a number of diopters more convergent than the sphere power. That means the spherical power describes the most divergent meridian and the cylindrical component describes the most convergent. In the minus cylinder noatation, the cylinder power is a number of diopters more divergent than the sphere component. In this convention, the sphere power describes the most convergent meridian and the cylinder component describes the most divergent. Europe typically follows the plus cylinder convention while the US minus cylinder notation is used by optometrists and plus cylinder notation is used by ophthalmologists.Minus cylinder notation is also more common in Asia, although either style may be encountered there. There is no difference in these forms of notation and it is easy to convert between them: simply add the sphere and cylinder numbers together to produce the converted sphere, then change the sign of the cylinder number and change the axis by 90º. For example, a lens with a vertical power of -3.75 and a horizontal power of -2.25 could be specified as either -2.25 -1.50 x 180 or -3.75 +1.50 x 090.

The axis defines the location of the sphere and cylinder powers. The name "axis" comes from the concept of generating a cylinder by rotating a line around an axis. The curve of that cylinder is 90° from that axis of rotation. When dealing with toric lenses, the axis defines the orientation of the steepest and flattest curvatures relative to horizontal and vertcal. The "3 o'clock" position is defined as zero, and the 90th meridian is a vertical line. A horizontal line passes through both zero and the 180th meridians. By convention, a horizontal axis is recorded as 180.

In a regular toric lens, the flattest and steepest curvatures are separated by 90°. As a result, the axis of the cylinder is also the meridian with the same power as the recorded sphere power. The cylinder power, as defined above is the power that is most different from the sphere power. Because they are defined relative to each other, it is important to know if the lens is being described in minus cylinder notation, where the sphere power is the most convergent / least divergent power. When using plus cylinder notation, the opposite is true.

If the lens is spherical (there is no cylinder component) then there is no need for an axis. A prescription like this is written with D.S. (diopters sphere) after the sphere power (e.g., −3.00 D.S.). This verifies that the prescription is truly spherical rather than the cylinder power being omitted in error.

• correction power is measured in diopters
• by convention, an axis of 90° is vertical, 0° or 180° are horizontal
• if the cylinder power is positive, the lens is most convergent 90° from the axis
• if the cylinder power is negative, the lens is most divergent 90° from the axis
• if the cylinder power is zero, the lens is spherical and has the same power in every meridian

A prescription of −1.00 +0.25 x 180 describes a lens that has a horizontal power of −1.00 D and a vertical power of −0.75.

Similar to medical prescriptions, eyeglass prescriptions are written on paper pads or included in a patient's electronic health record, and contain a number of different abbreviations and terms:

• DV is an abbreviation for distance vision. This specifies the part of the prescription designed primarily to improve far vision. In a bifocal lens, this generally indicates what is to be placed in the top segment.
• NV is an abbreviation for near vision. This may represent a single-vision lens prescription to improve near work, or the reading portion of a bifocal lens.
• OD is an abbreviation for oculus dexter, Latin for right eye from the patient's point of view. Oculus means eye.
• OS is an abbreviation for oculus sinister, Latin for left eye from the patient's point of view.
• OU is an abbreviation for oculi uterque, Latin for both eyes.

N.B.: In some countries, such as the United Kingdom, RE (right eye), LE (left eye), and BE (both eyes) are used. Sometimes, just right and left are used.

• SPH CYL and AXIS are values for describing the power of the lens using plus cylinder or minus cylinder notation.
• ADD is an abbreviation for Near Addition. This is the additional refractive power to be combined, or added, to the distance power to achieve the ideal near power.
• Prism and Base Prism refers to a displacement of the image through the lens, often used to treat strabismus and other binocular vision disorders. The prism value is measured in prism diopters, and Base refers to the direction of displacement.
• PD or IPD Pupillary Distance or Interpupillary Distance, respectively. It is the distance between pupil centers.
• BVD Back vertex distance is the distance between the back of the spectacle lens and the front of the cornea (the front surface of the eye). This is significant in higher prescriptions (usually beyond ±4.00D) as slight changes in the vertex distance for in this range can cause a power to be delivered to the eye other than what was prescribed.

The DV portion of the prescription describes the corrections for seeing far away objects. The NV portion is used in prescriptions for bifocals to see very close objects. For most people under forty years of age, the NV or near-vision portion of the prescription is blank because a separate correction for near vision is not needed. In younger people, the lens of the eye is still flexible enough to accommodate over a wide range of distances. With age, the lens hardens and becomes less and less able to accommodate. This is called "presbyopia"; the presby- root means "old" or "elder". (It is the same root as in the words priest and presbyterian.)

The hardening of the lens is a continuous process, not something that suddenly happens abruptly in middle age. Though it is typically by the middle age when the process has progressed to the point where it starts to interfere with reading. Therefore, almost everybody needs glasses for reading from the age of 40–45. Because young children have a wider range of accommodation than adults, they sometimes examine objects by holding them much closer to the eye than an adult would.

This chart (which is approximate) shows that a schoolchild has over ten diopters of accommodation, while a fifty-year-old has only two. This means that a schoolchild is able to focus on an object about 10 cm (3.9 in) from the eye, a task for which an adult needs a magnifying glass with a magnification of about 3.5.^[1]

The NV correction due to presbyopia can be predicted using the parameter age only. The accuracy of such a prediction is sufficient in many practical cases, especially when the total correction is less than 3 diopters.

When someone accommodates, they also converge their eyes. There is a measurable ratio between how much this effect takes place (AC:A ratio, CA:C ratio). Abnormalities with this can lead to many orthoptic problems.

The optical axis is the centre of a lens where light travels through and is not bent. The visual axis is where light travels through the eye to the retina and is essentially understood to not be bent.

Sometimes glasses are given with the optical axis shifted away from the visual axis. This creates a prismatic effect. Prisms can be used to diagnose and treat binocular vision and other orthoptics problems which cause diplopia such as:

• Positive and negative fusion problems
• Positive relative accommodation and negative relative accommodation problems

There is a surprising amount of variation in the way prescriptions are written; the layout and terminology used is not uniform.

When no correction is needed, the spherical power will sometimes be written as 0.00 and sometimes as plano (pl.). The lens, although not flat, is optically equivalent to a flat piece of glass, and has no refractive power.

When cylindrical correction is needed, the mathematics used to denote the combination of spherical and cylindrical power in a lens can be notated two different ways to indicate the same correction. One is called the plus-cylinder notation (or "plus cyl") and the other the minus-cylinder notation (or "minus cyl"), based upon whether the axis chosen makes the cylindrical correction a positive or negative number.

For example, these two prescriptions are equivalent:

The plus-cylinder notation shows the prescription as a correction of +2.00 diopters along an axis of 150° and an additional correction of +1.00 diopters, giving a total correction of (+2.00) + (+1.00) = +3.00 diopters, at 90 degrees from that meridian (= 60°).

The minus-cylinder notation shows the prescription as a correction of +3.00 diopters along an axis of 60° and an additional correction of −1.00 diopters, giving a total correction of (+3.00) + (−1.00) = +2.00 diopters, at 90 degrees from that meridian (= 150°).

The result in both cases is +2.00 diopters at the 150th meridian and +3.00 diopters at the 60th meridian.

The method to transform one format to another is called flat transposition:

• Add cylindrical value to the spherical one
• Invert the sign of cylindrical value
• Add 90° to axis value, and if the new axis value exceeds 180°, subtract 180° from the result

In practice, optometrists tend to use minus-cylinder notation, whereas ophthalmologists and orthoptists tend to prescribe using plus-cylinder notation. However, some ophthalmologists and orthoptists (such as in Australia) are changing to using minus-cylinder notation.^{[citation needed]}

In addition to the plus and minus cylinder notations, some countries use slight variations for special purposes. For example, the National Health Service of the United Kingdom uses the term Greatest Spherical Power when looking up the amount of state optical benefits that can apply to a particular prescription. This is simply the transposition of the prescription format so that the magnitude of the sphere is greatest. In the examples given earlier this would be the minus-cylinder version; that is, +3.00 −1.00 x 60° as opposed to +2.00 +1.00 x 150°.

• UK optical vouchers explained