Eyeglass Prescription Terms: The Complete Advanced Glossary

Eyeglass prescription terms are the clinical shorthand your optometrist uses to encode the precise optical correction your eyes require. Every abbreviation, number and symbol on that slip of paper has a specific, measurable meaning — and understanding each one is the difference between ordering confidently and guessing. This glossary defines every eyeglass prescription term you are likely to encounter, with exact values, units and cause-and-effect explanations throughout.

Why Eyeglass Prescription Terms Matter

Eyeglass prescription terms are not interchangeable or approximate — each one maps to a specific optical measurement that determines how your lenses are ground. Misreading a single term can result in lenses that correct the wrong eye, apply cylinder power at the wrong axis, or omit a prism correction entirely. For prescriptions above ±2.00 DS, a 10° axis error introduces measurable blur and spatial distortion. For prism prescriptions, a base direction error reverses the correction, actively worsening the condition it was meant to treat. Precision in reading your prescription is an optical safety requirement, not a preference.

Understanding your prescription also allows you to verify that the lenses you receive match what was prescribed. Every patient has the right to check their own prescription data against the delivered lenses using a lensometer reading, and knowing the terminology makes that verification possible. Learn how to read the full document in our eyeglass prescription guide.

Eyeglass Prescription Terms for Eye Designation — OD, OS, OU

The three eyeglass prescription terms for eye designation derive from Latin and appear on every prescription worldwide. OD stands for oculus dexter, meaning right eye. OS stands for oculus sinister, meaning left eye. OU stands for oculus uterque, meaning both eyes together, and appears primarily in binocular measurements such as pupillary distance or prism prescriptions where both eyes share the same value.

On a standard prescription form, OD always appears above OS. This ordering is universal and does not vary by country or prescribing system. A prescription that lists OS above OD has been formatted incorrectly and should be queried with the prescribing optometrist before ordering. Transposing OD and OS when entering prescription data is one of the most common ordering errors and results in lenses that correct each eye with the other eye's prescription.

Eyeglass Prescription Terms for Sphere and Cylinder

The sphere (SPH) and cylinder (CYL) values are the two core eyeglass prescription terms that define the primary refractive correction. Sphere is the spherical power of the lens, measured in dioptres (D), and corrects myopia (short-sightedness) when negative and hyperopia (long-sightedness) when positive. A sphere value of −3.00 D means the lens must converge light by 3 dioptres less than a flat lens to bring the focal point onto the retina of a myopic eye.

Cylinder is the additional power required to correct astigmatism — a condition where the cornea or lens has two different curvatures in perpendicular meridians. Cylinder power is always accompanied by an axis value, measured in degrees from 1 to 180, which specifies the orientation of the flat meridian of the cylinder. A CYL of −1.25 D at axis 090 means the lens requires 1.25 dioptres of additional minus power in the 90° meridian only. Without the axis, the cylinder value is optically meaningless.

Prescriptions can be written in either minus cylinder or plus cylinder form. The two forms are mathematically equivalent and can be transposed using a standard formula: reverse the sign of the cylinder, add the cylinder value to the sphere, and rotate the axis by 90°. Most optical laboratories work in minus cylinder form by default.

Eyeglass Prescription Terms for Reading and Near Vision — Add, NV, DV

Three eyeglass prescription terms relate specifically to near and reading vision. Add (addition) is the additional positive power added to the distance prescription to correct presbyopia — the age-related loss of the eye's ability to focus at near distances. Add values are always positive, typically ranging from +0.75 D to +3.50 D, and are identical for both eyes in the majority of cases. The Add value is added to the distance sphere to calculate the reading sphere: a distance prescription of −2.00 DS with an Add of +2.00 D produces a reading power of plano (0.00 D).

DV stands for distance vision and labels the portion of the prescription used for distance correction. NV stands for near vision and labels the reading portion. On bifocal and progressive prescriptions, both DV and NV sections appear. On single-vision distance prescriptions, only DV values are present. On single-vision reading prescriptions, the NV power is the sum of the DV sphere and the Add.

Progressive lenses require both a DV prescription and an Add value to calculate the full corridor of powers from distance through intermediate to near. A minimum Add of +0.75 D is generally required before a progressive lens design is optically justified. See our frame measurement guide for the minimum lens height requirements that progressive designs impose on frame selection.

Eyeglass Prescription Terms for Prism Correction

Prism is among the most specialised eyeglass prescription terms and appears only when the eyes have a binocular alignment problem — a condition where the two eyes do not point at the same object simultaneously without effort. Prism power is measured in prism dioptres (Δ), where 1Δ deflects a ray of light by 1 cm at a distance of 1 metre. A prism of 2Δ base-in on the right eye shifts the image 2 cm toward the nose at 1 metre, reducing the effort required for the eye to converge inward.

Base direction is the second required component of every prism prescription and specifies which edge of the prism is thickest. The four base directions are: Base In (BI) — thickest edge toward the nose, used for exophoria; Base Out (BO) — thickest edge toward the temple, used for esophoria; Base Up (BU) — thickest edge toward the top, used for hypotropia; Base Down (BD) — thickest edge toward the bottom, used for hypertropia. Reversing the base direction reverses the prismatic effect and worsens the condition. Read our dedicated prism glasses guide for a full clinical explanation.

Prism can be prescribed as horizontal, vertical, or oblique. Oblique prism is resolved into horizontal and vertical components using trigonometry before lens fabrication. Total prism in a prescription is sometimes split between the two eyes to reduce lens thickness asymmetry — a technique called prism splitting.

Eyeglass Prescription Terms for Pupillary Distance

Pupillary distance (PD) is the measurement in millimetres from the centre of one pupil to the centre of the other. It is the eyeglass prescription term that tells the laboratory where to place the optical centre of each lens within the frame. A PD error of 1 mm introduces approximately 0.33Δ of unwanted horizontal prism per dioptre of sphere power — for a −4.00 DS prescription, a 3 mm PD error introduces approximately 4Δ of induced prism, which is clinically significant and causes eyestrain and headaches.

PD is expressed in two ways. Binocular PD is the total distance between both pupils, typically 54–74 mm for adults. Monocular PD splits this into two measurements — right PD (RPD) and left PD (LPD) — each measured from the centre of the respective pupil to the centre of the nose bridge. Monocular PD is required for progressive lenses and for any prescription where the two eyes are not symmetrically positioned relative to the nose. Our PD measurement guide explains how to measure both binocular and monocular PD accurately at home.

A separate near PD is sometimes specified for reading glasses. Because the eyes converge when looking at near objects, near PD is typically 3–4 mm less than distance PD for the average adult interpupillary geometry.

Eyeglass Prescription Terms for Lens Geometry — Base Curve, Vertex Distance, OC

Three eyeglass prescription terms define the geometric relationship between the lens and the eye. Base curve (BC) is the curvature of the front surface of the lens, measured in dioptres of surface power. Base curve is selected by the laboratory to minimise off-axis aberrations for a given prescription power — it is not typically specified by the prescribing optometrist unless a specific lens form is clinically required. For most prescriptions, the laboratory selects the base curve automatically using established best-form lens tables.

Vertex distance (VD) is the distance in millimetres from the back surface of the lens to the front of the cornea, standardly measured at 12–14 mm. Vertex distance matters because the effective power of a lens changes with distance from the eye: a −10.00 DS lens at 12 mm vertex distance has a different effective power at the corneal plane than the same lens at 16 mm. For prescriptions above ±4.00 DS, a vertex distance change of 1 mm alters effective power by approximately 0.12 D — enough to require a compensated prescription if the frame sits significantly closer or farther than the trial frame used during the eye examination.

Optical centre (OC) is the point on the lens through which light passes without deviation. The laboratory positions the OC at the coordinates defined by the PD and fitting height measurements. A lens whose OC is not aligned with the pupil introduces unwanted prism in proportion to the decentration and the lens power. See our glasses fitting guide for the clinical impact of OC misalignment on visual performance.

Eyeglass Prescription Terms for Lens Materials and Index

Refractive index is the eyeglass prescription term that describes how efficiently a lens material bends light relative to air. A higher refractive index produces a thinner lens at equivalent prescription power because the material bends light more steeply, requiring less curvature — and therefore less thickness — to achieve the same optical effect. Standard plastic (CR-39) has a refractive index of 1.50. Mid-index lenses are 1.56 or 1.60. High-index lenses are 1.67 or 1.74.

The practical thickness reduction is significant: a −6.00 DS prescription in 1.50 index produces a centre thickness of approximately 1.0 mm and an edge thickness of approximately 9–10 mm in a 52 mm lens. The same prescription in 1.74 index reduces edge thickness to approximately 5–6 mm — a reduction of approximately 40%. FuzWeb carries the full MR lens series through the Bobbie collection: MR-8 (1.60 index), MR-7 (1.67 index), and MR-10 (1.74 index).

Abbe value is a secondary lens material term that measures chromatic aberration — the degree to which a lens splits white light into its component colours at the periphery of the lens. CR-39 has an Abbe value of 58 (low chromatic aberration). High-index 1.74 materials have an Abbe value of approximately 33 (higher chromatic aberration). For prescriptions above ±6.00 DS, Abbe value becomes a clinically relevant factor in peripheral visual quality. All FuzWeb lenses include UV400 protection, HMC (Hard Multi-Coat), anti-reflection coating, hydrophobic treatment, and oleophobic treatment on both surfaces as standard.

How to Use Eyeglass Prescription Terms When Ordering at FuzWeb

Applying eyeglass prescription terms correctly when ordering online requires transferring each value from your prescription to the order form without transposition or rounding. Enter OD values in the right eye fields and OS values in the left eye fields — never reverse them. Enter sphere and cylinder values with their signs (+ or −) exactly as written. Enter the axis as a whole number between 1 and 180 — do not add a degree symbol or leading zeros unless the form requires them. Enter PD as either a single binocular value or as two monocular values (RPD and LPD) depending on what your prescription specifies.

If your prescription includes prism, contact FuzWeb before ordering — prism lenses require specialist fabrication and are not available through the standard online ordering flow. For all other prescriptions, follow the FuzWeb 6-step lens ordering process to submit your prescription data correctly. Frame material affects how the finished lenses sit relative to your eyes — see our frame materials guide for how titanium, alloy, and acetate frames differ in dimensional stability and their effect on vertex distance consistency over time.

If you are unsure about any eyeglass prescription term on your prescription, our prescription reading guide walks through a complete sample prescription field by field. For lens selection by prescription power and index, our titanium eyeglasses guide and semi-rimless guide both include prescription compatibility sections.

FAQ About Eyeglass Prescription Terms

What does SPH mean on an eyeglass prescription?

SPH stands for sphere and is the primary power of the lens measured in dioptres. A negative SPH value corrects myopia (short-sightedness) by diverging light before it enters the eye. A positive SPH value corrects hyperopia (long-sightedness) by converging light. A value of 0.00 or the abbreviation PL (plano) means no spherical correction is required in that meridian.

What is the difference between OD and OS on a prescription?

OD (oculus dexter) designates the right eye and OS (oculus sinister) designates the left eye. On all standard prescription forms, OD appears above OS. These Latin abbreviations are universal across all countries and prescribing systems. Transposing OD and OS when ordering lenses results in each eye receiving the other eye's correction.

What does the axis number mean on an eyeglass prescription?

The axis is the orientation of the cylinder correction, measured in degrees from 1 to 180. It specifies the meridian of the eye that requires no additional cylinder power — the flat meridian of the toric lens surface. An axis of 090 means the cylinder power is applied in the horizontal meridian. The axis value is only present when a CYL value is also present; without cylinder power, axis is not applicable.

What does Add mean on an eyeglass prescription?

Add (addition) is the additional positive power added to the distance prescription to correct presbyopia — the age-related loss of near focusing ability. Add values are always positive and typically range from +0.75 D to +3.50 D. The Add is added to the distance sphere to calculate the reading power. An Add value appears only on prescriptions for bifocal, trifocal, or progressive lenses, or for dedicated reading glasses.

What is a prism dioptre on an eyeglass prescription?

A prism dioptre (Δ) is the unit of prismatic power that deflects a ray of light by 1 centimetre at a distance of 1 metre. Prism is prescribed to correct binocular alignment problems where the eyes do not point at the same object simultaneously without muscular effort. Every prism prescription must include both a power in prism dioptres and a base direction (Base In, Base Out, Base Up, or Base Down) — the power without the base direction is optically incomplete.

What does refractive index mean for eyeglass lenses?

Refractive index is a dimensionless number that describes how efficiently a lens material bends light relative to air (index 1.00). A higher refractive index produces a thinner lens at the same prescription power because the material requires less curvature to achieve the same optical effect. Standard CR-39 plastic has an index of 1.50. A 1.74 high-index lens is approximately 40% thinner than a 1.50 lens at equivalent prescription power and lens diameter.

What is vertex distance and why does it matter?

Vertex distance is the measurement in millimetres from the back surface of the lens to the front of the cornea, standardly 12–14 mm. It matters because the effective power of a lens at the corneal plane changes as vertex distance changes. For prescriptions above ±4.00 DS, a 1 mm change in vertex distance alters effective corneal power by approximately 0.12 D. Frames that sit significantly closer or farther from the eye than the trial frame used during the examination require a compensated prescription to maintain the intended correction.

Know Your Terms, Order with Confidence

Eyeglass prescription terms are a precise, standardised language developed to eliminate ambiguity in optical correction. Every abbreviation has a single definition, every unit has a fixed scale, and every value has a direct consequence for how your lenses are fabricated and how they perform on your face. OD is always the right eye. Cylinder without axis is always incomplete. Prism without base direction is always unorderable. These are not conventions — they are optical facts.

The practical benefit of understanding this language is control: you can verify your prescription before ordering, confirm your PD measurement independently, identify errors before lenses are cut, and communicate precisely with your optometrist or optician when something does not seem right. Use the FuzWeb 6-step ordering process to apply these terms correctly, and consult our PD guide and prescription reading guide for the two measurements most commonly entered incorrectly when ordering online.