You've just had an eye exam and you're holding a piece of paper covered in abbreviations, numbers, and symbols that look like a foreign language. You're not alone — eyeglass prescriptions confuse almost everyone the first time. This guide breaks down exactly what each number and abbreviation means, so you can understand your prescription with confidence.
The Basic Structure of a Glasses Prescription
A standard glasses prescription contains measurements for each eye, typically arranged in a table with columns for different values. You'll see abbreviations for each eye:
- OD (Oculus Dexter) — your right eye
- OS (Oculus Sinister) — your left eye
- OU (Oculus Uterque) — both eyes together (less common)
The main columns you'll see are SPH, CYL, Axis, and sometimes ADD and Prism. Let's go through each one.
SPH (Sphere): Your Basic Vision Correction
The Sphere value is the most fundamental part of your prescription. It indicates the lens power needed to correct your distance vision, measured in dioptres (D).
- Negative numbers (e.g. -2.50) indicate myopia (short-sightedness) — you struggle to see things far away
- Positive numbers (e.g. +1.75) indicate hyperopia (long-sightedness) — you struggle to see things up close
- The higher the number (regardless of sign), the stronger the prescription
A prescription of -0.25 or +0.25 is very mild. A prescription of -6.00 or higher is considered strong myopia.
CYL (Cylinder): Astigmatism Correction
The Cylinder value corrects for astigmatism — an irregularity in the curvature of your cornea or lens that causes blurred or distorted vision at all distances. Like Sphere, it's measured in dioptres and can be positive or negative.
- If this column is blank or marked "DS" (Dioptre Sphere), you have no significant astigmatism
- A small CYL value (e.g. -0.25 or -0.50) indicates mild astigmatism
- A larger value (e.g. -2.00 or more) indicates more significant astigmatism
Axis: The Direction of Astigmatism
If you have a CYL value, you'll also have an Axis value. This number, between 1 and 180, indicates the orientation (angle) of the astigmatism correction in degrees. It tells the lens maker which direction to orient the cylindrical correction.
The Axis number on its own means nothing without the CYL value — they always work together.
ADD: Reading Addition (for Progressive or Bifocal Lenses)
The ADD value appears on prescriptions for people who need different corrections for near and distance vision — typically those experiencing presbyopia (age-related difficulty focusing up close). It represents the additional magnifying power added to the lower portion of a progressive or bifocal lens.
- ADD values are always positive, typically ranging from +0.75 to +3.50
- The same ADD value usually applies to both eyes
- If you have an ADD value, you'll likely need progressive lenses or separate reading glasses
If you have an ADD value and prefer not to wear bifocals or progressives, a separate pair of reading glasses is a practical alternative for close-up tasks.
Prism: Eye Alignment Correction
Prism correction is less common and appears only when there's a muscle imbalance causing the eyes to point in slightly different directions. It's measured in prism dioptres and includes a direction (Base Up, Base Down, Base In, Base Out).
Most people will never have a prism value on their prescription.
PD: Pupillary Distance
Pupillary Distance (PD) is the measurement in millimetres between the centres of your pupils. It's essential for positioning the optical centre of each lens correctly in front of your eye. Without an accurate PD, even a perfect prescription can cause visual discomfort.
- Single PD: one measurement for both eyes combined (typically 58–68mm for adults)
- Dual PD: separate measurements for each eye (e.g. 32/31), which is more precise
Your PD may or may not be included on your prescription — if it's not, ask your optician to measure it.
A Sample Prescription Explained
Here's what a typical prescription might look like and what it means:
| SPH | CYL | Axis | ADD | |
|---|---|---|---|---|
| OD (Right) | -2.25 | -0.75 | 180 | +1.50 |
| OS (Left) | -1.75 | -0.50 | 165 | +1.50 |
Reading this prescription: the person is moderately short-sighted in both eyes (SPH values), has mild astigmatism in both eyes (CYL values), and needs reading addition for close-up work (ADD value) — suggesting they'd benefit from progressive lenses.
How Long Is a Prescription Valid?
Prescription validity varies by location, but is typically one to two years for adults. Your vision can change over time, so it's important to have regular eye examinations and update your prescription accordingly. Wearing an outdated prescription can cause eye strain, headaches, and reduced visual clarity.
What Your Prescription Doesn't Tell You
Your prescription specifies the optical correction you need, but it doesn't determine which frames or lens materials are right for you. Those choices depend on your lifestyle, face shape, aesthetic preferences, and the advice of your optician.
Once you have your prescription, the next step is finding frames that complement your face and suit your daily life. Explore our full eyewear collection to find premium frames in titanium and acetate, or browse our Best Selling styles for tried-and-tested favourites.
When to See Your Optician
If you're unsure about any aspect of your prescription, always consult your optician. They can explain your specific values, recommend appropriate lens types, and ensure your new glasses are made to the correct specification. A prescription is a medical document — understanding it is empowering, but professional guidance ensures you get the best possible outcome.
