Eye Prescription Explained: How to Read and Understand Your Glasses Prescription

Understanding eye prescription details can feel confusing at first. Numbers, abbreviations, plus and minus signs and unfamiliar Latin terms often leave people asking: what does my eye prescription mean? This guide breaks everything down clearly, helping you understand your glasses prescription, what each number means, and how it affects your vision correction.

Whether you are ordering glasses online, replacing lenses, or simply reviewing a replacement copy from your optician, this guide to glasses prescription explained will help you feel confident about your eye health and vision needs.

What Is an Eye Prescription?

An eye prescription is a written record of the lens power needed to correct your vision following an eye examination. It tells you exactly how lenses should be made to give you clear vision at different distances, whether for distance vision, near vision, or both.

Your prescription is based on visual acuity testing, eye health checks, and how light focuses inside your eyes. Not all prescriptions look the same, and different prescriptions are used for glasses and contact lenses.

Glasses Prescription vs Contact Lens Prescription

A glasses prescription and a contact lens prescription are not interchangeable. Although both are designed to improve vision, they differ in several ways:

• Glasses sit away from the eye, while contact lenses rest directly on the eye’s surface
• Contact lens prescriptions include base curve and diameter values
• Glasses prescriptions use vertex distance instead

This difference matters because the amount of lens power needed changes depending on how close the lens sits to the eye. Always make sure you are using the correct prescription when ordering contact lenses or glasses online.

Understanding Left and Right Eyes on Your Prescription

Your prescription is split into two rows or columns for each eye:

• Right eye (OD – oculus dexter)
• Left eye (OS – oculus sinister)

Sometimes you may also see OU, which refers to both eyes together. The left eye OU wording can confuse people, but os refers specifically to the left eye, while ou refers to both.

It is very common to have different prescriptions for left and right eyes, as most people’s eyes do not focus light in exactly the same way.

Sphere (SPH): The Main Lens Power

Sphere, often written as SPH, shows the main lens power needed for vision correction.

• A minus sign indicates short sightedness (myopia)
• A plus sign indicates long sightedness (hyperopia)

The higher the number, the stronger the prescription. A higher the number value means more lens power is required to bring images into focus. This applies whether the number is positive or negative.

If this box is blank or shows an empty box, it usually means no spherical correction is needed for that eye.

Short Sightedness and Long Sightedness Explained

Short sighted people see close up objects clearly but struggle with distant objects. Long sighted people often see distant objects more clearly but may struggle with close up objects, particularly with age.

Long sightedness may also cause eye strain, headaches, or blurred vision during reading. Both conditions are extremely common and easily corrected with glasses or contact lenses.

Cylinder (CYL) and Astigmatism

Cylinder power corrects astigmatism, which occurs when the eye is not perfectly spherical. Instead of being completely round, the eye has an irregular curve that causes light to focus unevenly.

Astigmatism can affect both near and distance vision and may cause:

• Blurred or distorted vision
• Eye strain
• Headaches

The cyl number shows how much cylindrical power is needed. The higher the number, the stronger the astigmatism correction required.

Axis: Direction of Astigmatism

Axis works alongside cylinder power and is measured in degrees from 0 to 180. It shows the direction of the irregular curve in your eye.

Axis only appears if you have astigmatism. Without cylinder power, axis is not needed.

Add Power: Near Vision and Reading Support

Add power shows the added lens power needed for near vision tasks like reading. This is most common in people over 40 and is linked to presbyopia.

Add values are used in:

• Varifocal lenses
• Progressive lenses
• Bifocals

The added lens power needed helps your eyes focus comfortably at different distances without switching between two prescriptions.

Single Vision, Varifocal and Progressive Lenses

Single vision lenses correct one distance only, either near or far. They are commonly used for everyday glasses and reading glasses. You can explore options for single vision lenses when ordering new glasses or replacing lenses.

Varifocal lenses and progressive lenses combine distance, intermediate and near vision in one lens, with no visible line. The bottom half of the lens is typically used for close work, while the top supports distance vision.

Prism Correction and Double Vision

Prism correction is used when there is a muscle imbalance that causes double vision. Instead of correcting focus, prism lenses help align images correctly between the eyes.

Prism diopters show how much correction is required. This type of additional correction can help prevent double vision and improve comfort.

Not all prescriptions include prism values, and they are only added when clinically necessary.

Pupillary Distance (PD) and Centre Point

Pupillary distance measures the distance between the centre point of your pupils. This ensures lenses are centred correctly in the frame, allowing for best vision and comfort.

PD is especially important when ordering glasses online, as incorrect measurements can affect visual clarity and cause discomfort.

Vertex Distance and Back Vertex Distance

Vertex distance refers to the space between your eye and the back surface of the lens. Back vertex distance becomes important for stronger prescriptions, where small changes can affect how light enters the eye.

Opticians account for vertex distance during eye tests to ensure accurate prescription values.

What Do All the Numbers Mean on My Glasses?

Many people ask what the numbers mean on my glasses. Each number represents a specific optical measurement that contributes to your overall vision correction.

The prescription tells:

• How much lens power is required
• Whether astigmatism is present
• If additional correction is needed for reading
• Whether prism is required

Understanding your prescription explained in simple terms helps you make informed choices about lenses and frames.

Prescription Explained: Common Abbreviations

Here are some common abbreviations you may see:

• SPH: spherical power
• CYL: cylindrical power
• AXIS: direction of astigmatism
• ADD: near vision support
• OD: right eye
• OS: left eye
• OU: both eyes

These latin terms are standard across optical prescriptions worldwide.

Two Prescriptions and Different Distances

You may receive two prescriptions if you need separate glasses for reading and distance. Some people prefer this, while others choose varifocal lenses for convenience.

Different distance corrections depend on lifestyle, comfort and visual needs.

Eye Test vs Eye Health Examination

An eye test measures how well you see, while an eye health examination checks the overall health of your eyes. Both are essential for maintaining healthy vision.

Regular eye examinations help detect early signs of conditions such as glaucoma, macular degeneration and diabetes-related eye changes.

Ordering Glasses Online With Confidence

Ordering glasses online is safe and convenient when you understand your prescription. Make sure you:

• Enter details exactly as written
• Use the correct prescription type
• Include pupillary distance

If you already own frames you love, you can reglaze glasses rather than buying a completely new pair.

Why Prescriptions Change Over Time

Your vision can change due to age, health, screen use, or lifestyle. This is why regular eye examinations are important.

A new pair of glasses ensures your lenses match your current visual needs, helping maintain comfort and clarity.

Not All Prescriptions Are the Same

Not all prescriptions follow identical formats. Some opticians include additional notes or measurements, while others keep layouts simple.

Always ask for a replacement copy if you misplace your prescription, and check expiry dates before ordering lenses.

How Prescription Strength Affects Lens Choice

The amount of lens power affects lens thickness, weight and design. Higher prescriptions may benefit from thinner lenses or specialised materials.

Your optician or online retailer can advise on the best lens options for comfort and appearance.

Understanding Optical Prescription Explained Simply

Understanding optical prescription details empowers you to make better decisions about your eyewear. Once you know what each value represents, reading prescriptions becomes straightforward rather than intimidating.

When to Ask for Help

If anything on your prescription is unclear, ask your optician to explain it. Understanding your eye prescription supports better eye health and ensures you get the best vision possible.

Final Thoughts: Understanding Your Eye Prescription

Understanding your eye prescription is essential for maintaining clear vision, comfort and long-term eye health. Whether you are choosing single vision lenses, varifocals or reglazing existing frames, knowing what each number means helps you order with confidence.

With a clear understanding of glasses prescription explained, you can take control of your vision and make informed choices that support healthy vision for years to come.

Frequently Asked Questions About Eye Prescriptions

How long is an eye prescription valid for?

In the UK, a glasses prescription is usually valid for two years. Contact lens prescriptions are often valid for one year, as they require more frequent checks.

Can I use an old prescription to order glasses?

You can, but it’s not recommended if your prescription is out of date. Vision can change gradually, and wearing the wrong prescription may cause headaches or eye strain.

Why is my left eye prescription different from my right eye?

It’s very common for each eye to need different levels of correction. Most people’s eyes do not focus light in exactly the same way.

What does a high eye prescription mean?

A higher prescription means stronger lens power is required to correct your vision. This can affect lens thickness and may influence the type of lens material recommended.

What if my prescription has no CYL or AXIS values?

If there is no CYL or AXIS, it means you do not have astigmatism and do not require cylindrical correction.

Is pupillary distance written on my prescription?

Not always. Some opticians provide PD separately, especially for online orders. You may need to measure it yourself if it is not included.

Can I order glasses online with just my prescription?

Yes, as long as you have all required details, including SPH, CYL (if applicable), AXIS, ADD (if needed), and pupillary distance.

What does ADD mean on my glasses prescription?

ADD refers to additional lens power for near vision, commonly used in varifocal, progressive or bifocal lenses for reading.

Why does my contact lens prescription look different?

Contact lenses sit directly on the eye, so they require different measurements such as base curve and diameter. Glasses and contact lens prescriptions are not interchangeable.

Should I choose single vision or varifocal lenses?

This depends on your lifestyle and vision needs. Single vision lenses correct one distance, while varifocals allow clear vision at multiple distances without changing glasses.