How to Read an Optical Prescription
Understanding your glasses prescription, explained simply.
Have you ever looked at your optical prescription and wondered what all the numbers and abbreviations mean? From SPH and CYL to AXIS and ADD, each measurement provides important information about the lenses prescribed to correct your vision.
Our easy-to-follow guide explains the most common terms found on an optical prescription, helping you understand what they mean and how they relate to your glasses.
Image suggestion: A high-quality photograph of modern spectacles alongside an eye chart, with the heading overlaid on a clean background.
What Is an Optical Prescription?
An optical prescription is provided following an eye examination and specifies the lens power required to correct your refractive error. It may include additional measurements to help ensure your glasses are made and fitted appropriately.
Your prescription may correct one or more of the following:
- Short-sightedness (myopia): Distant objects appear blurred.
- Long-sightedness (hyperopia): Close-up objects may be difficult to focus on.
- Astigmatism: Vision may appear blurred or distorted due to differences in the curvature of the eye’s optical surfaces.
- Presbyopia: Age-related difficulty focusing on nearby objects.
Understanding the Numbers on Your Prescription
An optical prescription may appear complicated at first, but each abbreviation has a specific purpose.
Image suggestion: An annotated example prescription with SPH, CYL, AXIS and ADD highlighted in different colours.
SPH – Sphere
Sphere indicates the main lens power required to correct short-sightedness or long-sightedness. It is measured in dioptres (D).
- Minus (−): Usually indicates a correction for short-sightedness.
- Plus (+): Usually indicates a correction for long-sightedness.
- Zero or plano (PL): Indicates no spherical correction is required.
For example, a prescription of −2.00 D indicates a minus spherical correction, while +2.00 D indicates a plus spherical correction.
CYL – Cylinder
Cylinder indicates the amount of lens correction required for astigmatism.
Astigmatism can cause blurred or distorted vision at different distances. If your prescription includes a CYL value, your lenses require a specific amount of astigmatism correction.
AXIS – Axis
Axis specifies the orientation of the astigmatism correction and is measured in degrees, usually from 1° to 180°.
The axis works alongside the CYL value to ensure the lens is manufactured with the appropriate orientation.
Remember: The AXIS value describes direction, not the severity of astigmatism.
ADD – Addition
ADD refers to the additional plus power used to support near vision, such as reading or other close-up tasks.
It is commonly found on prescriptions for people experiencing presbyopia and may be used when prescribing reading glasses, bifocals or varifocal (progressive) lenses.
For example, an ADD of +1.50 D represents additional near power.
What Do OD and OS Mean?
You may notice these abbreviations on your prescription:
- OD (Oculus Dexter): Right eye.
- OS (Oculus Sinister): Left eye.
- OU (Oculus Uterque): Both eyes.
These traditional Latin abbreviations identify which eye each prescription measurement relates to.
What Is Pupillary Distance (PD)?
Pupillary distance, or PD, is the distance between the centres of your pupils, measured in millimetres. Some prescriptions provide one measurement for both eyes, while others list a separate measurement for each eye.
PD helps ensure your lenses are positioned appropriately in relation to your pupils when your glasses are made.
Image suggestion: A simple, labelled illustration showing how pupillary distance is measured.
What Does Prism Mean on an Optical Prescription?
Prism may be prescribed to help manage certain eye alignment or binocular vision difficulties.
It is measured in prism dioptres and may be accompanied by a direction, such as base-in, base-out, base-up or base-down.
Prism is not required for every prescription. If your prescription includes a prism value, your optometrist or dispensing optician can explain its purpose.
How Does Your Prescription Affect Your Glasses?
Your prescription helps determine the optical power of your lenses, but choosing the right glasses involves more than just the numbers.
Your optician may also consider:
- Your frame size, shape and fit.
- Lens material and thickness.
- Whether single-vision, bifocal or varifocal lenses are appropriate.
- Your lifestyle and everyday visual requirements.
- Lens coatings and other available features.
The aim is to provide lenses that meet your prescription requirements and suit your individual needs.
Frequently Asked Questions
Does a minus prescription mean my eyesight is worse?
No. Minus and plus values indicate different types of optical correction. The sign alone does not determine your overall eye health or the quality of your vision.
Does a higher prescription mean stronger glasses?
Generally, a larger absolute lens power means more optical correction is required. However, prescription strength alone does not tell you everything about your eye health or visual ability.
Can my optical prescription change?
Yes. Your prescription can change over time due to age and other factors. Regular eye examinations help monitor changes in your vision and assess your eye health.
Can I use my glasses prescription to order contact lenses?
Not necessarily. Contact lenses require a separate prescription that may include additional measurements, such as base curve and diameter. A suitable fitting is important to ensure that contact lenses fit correctly and are appropriate for your eyes.
What does PL mean on a prescription?
PL stands for plano and indicates zero spherical lens power. Other corrections, such as cylinder power for astigmatism, may still be required.
Need Help Understanding Your Prescription?
Understanding your optical prescription is an important step towards making informed decisions about your eyewear.
If you have questions about your prescription or need help choosing suitable lenses, speak to your optometrist or dispensing optician for advice tailored to your individual needs.
Book an eye examination or contact our optical team today to find out more about your vision and eyewear options.
Educational disclaimer: This article is intended for general information only and does not replace a professional eye examination or individual optical advice. Prescription requirements and lens suitability should be confirmed by an appropriately qualified eye care professional.