An eyeglass prescription can look like a collection of abbreviations, plus and minus signs, and numbers that are difficult to interpret at first. Once you know what each field represents, the prescription becomes easier to understand. SPH describes the main focusing power, CYL and AXIS describe astigmatism correction, ADD is an additional near power, and PD helps position the lenses in front of your eyes.

This guide explains common fields on a United States eyeglass prescription, shows a fictional example, and explains what the numbers do and do not tell you. Prescription layouts vary between practices, and not every prescription includes every field. Use this information to understand the document, not to diagnose an eye condition or choose a prescription without a qualified eye care professional.

A quick guide to the numbers on your prescription

Prescription field	What it generally means
OD	Right eye
OS	Left eye
SPH or Sphere	Main lens power for nearsightedness or farsightedness
CYL or Cylinder	Amount of astigmatism correction, when present
AXIS	Direction of the cylinder correction, written in degrees
ADD	Additional plus power used for near tasks in multifocal prescriptions
PD	Pupillary distance used to position lenses
Prism and Base	A prescribed prism amount and its orientation, when needed

These values work together. Do not compare one number in isolation or assume that a larger number always means a particular level of visual disability. The meaning depends on the field, its sign, the other values, and the clinical context.

Example: how an eyeglass prescription may look

The following is a fictional example for learning purposes only. It is not a recommendation for any real person and must not be used to order glasses.

Eye	SPH	CYL	AXIS	ADD
OD (right)	-2.00	-0.75	180	+1.50
OS (left)	-1.50	-1.00	170	+1.50

In this example, both eyes have negative sphere values, which commonly indicates correction for nearsightedness. Both have cylinder values, meaning the prescription includes astigmatism correction, and each cylinder value has an axis direction. The ADD value indicates extra plus power for near vision in a multifocal lens design. Your own prescription may use different numbers, omit fields, or use plus-cylinder notation instead of minus-cylinder notation.

OD and OS: which eye is which?

OD stands for oculus dexter, the right eye. OS stands for oculus sinister, the left eye. Some prescriptions also use OU, meaning both eyes. The eye labels help keep each set of lens values associated with the correct eye. Check the labels carefully when reading a prescription or discussing an order with an optical professional.

Some prescription forms list the right eye first and the left eye second, but the layout can vary. Never assume that the first column always refers to a particular eye if the headings are unclear. Ask the prescribing practice or optician to clarify any ambiguous field before ordering lenses.

SPH: what sphere means

SPH is short for sphere. It describes the main lens power used to correct focusing error. It is measured in diopters and is commonly written with a plus sign or a minus sign. A minus sphere value is generally used to correct myopia, also called nearsightedness, where distant objects may look blurry. A plus sphere value is generally used to correct hyperopia, also called farsightedness, although the way hyperopia affects vision can vary by age and degree.

The further a sphere value is from zero, the greater the prescribed spherical lens power. However, the number alone does not tell you how well you see without glasses, how healthy your eyes are, or how severe an eye condition may be. Visual acuity, eye health, age, accommodation, and other prescription components all matter.

A value of plano, sometimes written as PL or 0.00, means no spherical power is prescribed in that field. It does not necessarily mean that the entire prescription is zero, because CYL, prism, or other values may still be present.

CYL: what cylinder means

CYL is short for cylinder. It records the amount of lens power used to correct astigmatism, a common refractive condition in which the eye does not focus light equally in all meridians. Astigmatism can make vision blurred or distorted at distance, near, or both, depending on the prescription and the person.

Cylinder may be written as a negative or positive number. Both notations can describe equivalent optical corrections when the other prescription values are converted correctly. This is called transposition. Do not try to change a plus CYL value to minus CYL, or vice versa, by editing just one field. A valid transposition also changes the sphere value and the axis.

If the CYL field is blank, marked with a dash, or shown as zero, it may mean that no cylinder correction is prescribed in that format. But paperwork conventions vary, so ask the issuing practice if you are unsure whether a field was intentionally left blank.

AXIS: why the degree value matters

AXIS specifies the orientation of the cylinder correction and is written as a number from 1 to 180 degrees. It is not a measure of how much astigmatism you have. The amount of astigmatism is represented by CYL; AXIS tells the lens maker the direction in which that cylinder power must be placed.

For example, an axis of 180 and an axis of 90 refer to different orientations. Neither is automatically stronger or worse than the other. The axis only has meaning alongside a cylinder value. If a prescription includes no cylinder correction, an axis may be omitted or left blank.

Because axis orientation matters to lens performance, an incorrect axis can affect clarity and comfort. If glasses made from a new prescription seem wrong, do not try to adjust the axis yourself. Ask the optical practice to verify the written prescription, the finished lenses, and the way the frame fits.

ADD: additional power for near vision

ADD means addition or near addition. It is the extra plus power used in multifocal prescriptions to support near tasks such as reading. It is commonly seen on prescriptions for bifocal, trifocal, or progressive lenses, and is often associated with presbyopia, the age-related reduction in the eye’s ability to focus up close.

An ADD value is usually written as a positive number. It is added to the distance prescription to determine the near power, with the exact calculation and lens design handled by the optical professional. A prescription may show one ADD value for both eyes or separate values depending on the form and clinical needs. Do not assume every prescription with an ADD requires the same lens design.

ADD is not the same as the full near prescription, and it does not tell you which frame or progressive lens design you should buy. Working distance, lens design, fitting height, frame size, and your visual needs can affect the final choice. Discuss how you use your glasses, such as reading, computer work, driving, or switching between tasks.

PD: pupillary distance and lens positioning

PD stands for pupillary distance, the distance between the centres of the pupils. It helps the optical professional position the lenses so their optical centres line up appropriately with your eyes. PD may be recorded as one binocular measurement, such as 64 mm, or as two monocular measurements, such as 32 mm for each eye. Monocular measurements can help account for natural differences between the two sides of the face.

PD is a measurement in millimetres, not a lens power. It is not the same as the distance between the outer edges of the eyes or the width of the eyeglass frame. Depending on the lens type and prescription, additional measurements such as fitting height may be required, especially for progressive lenses.

Not every prescription document prints PD because it may be measured or recorded separately by the dispensing optical practice. If you are ordering glasses online, follow the retailer’s measurement instructions and use an accurate measurement from a qualified source when needed. Do not guess a PD from a photo or use a generic average when precise fitting measurements are required.

Prism and base direction

Some prescriptions include prism, which is used when clinically indicated to help manage certain binocular vision or eye alignment issues. Prism may be expressed in prism diopters, often written as a number followed by a triangle-like symbol or the word prism. The base direction may be written as base up, base down, base in, or base out, or abbreviated on the form.

Prism is different from SPH and CYL and should not be interpreted as a stronger or weaker ordinary prescription. If your prescription includes prism, make sure the value and base direction are copied accurately. If any notation is unclear, ask the prescriber or optician to explain it before placing an order.

What does a glasses prescription say about 20/20 vision?

A glasses prescription describes the lens correction needed to improve focusing, but it does not directly state your visual acuity or give a complete picture of eye health. Some people achieve 20/20 vision with correction, while others may not because of eye disease, amblyopia, irregular corneal shape, or other factors. The numbers alone cannot tell you whether your eyes are healthy.

A prescription is also not a substitute for a comprehensive eye examination. If you have new or sudden vision changes, eye pain, flashes, a sudden increase in floaters, a curtain-like shadow, or other concerning symptoms, seek prompt professional assessment rather than trying to interpret the prescription on your own.

Why a contact lens prescription is different

An eyeglass prescription should not be used as a contact lens prescription. Glasses sit a short distance in front of the eyes, while contact lenses sit directly on the tear film over the cornea. Contact lens prescribing also involves fit, lens material, design, and other details that are not represented by a standard glasses prescription. A contact lens prescription may include base curve, diameter, brand or lens type, and power.

Contact lenses require their own fitting and follow-up process. Even if the power looks similar, the prescription is not interchangeable. Ask an eye care professional for a current contact lens prescription and guidance about safe wear and replacement.

How prescription numbers affect your choice of frames and lenses

The prescription is one part of choosing eyewear. Lens power, cylinder, frame size, lens shape, material, and how the frame sits on your face can all influence thickness, weight, appearance, and optical performance. Stronger prescriptions may benefit from careful frame and lens selection, but there is no single frame shape or lens material that is best for everyone.

For example, a smaller frame can sometimes reduce lens edge thickness for some prescriptions, while a high-index lens material may help make lenses thinner. These are trade-offs, not universal rules. Optical centre placement, face measurements, lens design, durability, and budget should also be considered. Learn about options through the Lumiere Optique guide to high-index lenses and discuss your individual prescription with a qualified optical professional.

Questions to ask your optician

• Can you explain the SPH, CYL, AXIS and ADD values on my prescription?
• Is the cylinder written in plus-cylinder or minus-cylinder format?
• Do I need a PD measurement or other fitting measurements for the lens type I am choosing?
• Does my prescription include prism or any special lens requirements?
• Which lens materials and frame sizes are suitable for my prescription and priorities?
• When does my prescription expire, and should I have another eye examination before ordering?

If any number is missing, unclear, or appears different from what you expected, contact the office that issued the prescription. A qualified professional can confirm what the entries mean and whether additional measurements are needed. Do not copy values from an old pair of glasses as a substitute for a current prescription.

Frequently asked questions

Does a higher negative SPH number mean worse eyesight?

A more negative SPH value generally means more minus lens power is prescribed, often to correct a greater degree of myopia. It does not, by itself, measure overall eye health or determine how well a person sees with correction.

What if CYL or AXIS is blank?

It may mean that no cylinder correction is recorded, but forms differ. AXIS is relevant when a cylinder value is prescribed. If the notation is unclear, confirm it with the issuing practice before ordering glasses.

Why do some prescriptions have two ADD values?

Some prescriptions may list separate near additions for the right and left eyes or use a format with different near tasks. Ask the prescriber or optician to clarify the specific form rather than assuming the values are interchangeable.

What if my prescription does not include PD?

PD may be measured or recorded separately by the optical practice and is not printed on every prescription. Ask whether it is needed for your glasses order and have it measured accurately if required.

Can I use my eyeglass prescription to order contact lenses?

No. Contact lenses require a separate prescription and fitting because they sit directly on the eye and involve fit and lens-specific parameters in addition to power.

Does this prescription tell me whether my eyes are healthy?

No. It describes optical correction, not a complete assessment of eye health. Regular eye examinations and prompt evaluation of new symptoms remain important.

Explore prescription eyewear with professional fitting

Understanding your prescription can make it easier to ask informed questions and compare eyewear options, but the correct lens order depends on the full prescription and accurate fitting measurements. Do not self-adjust the values or use this guide as a replacement for professional advice.

To explore frames and lens options, visit Lumiere Optique’s eyewear collections or learn more about an eye exam in Scottsdale. A qualified eye care professional or optician can help explain your prescription and the measurements needed for your chosen lenses.