Getting a new prescription from your optometrist or eye doctor can be exciting, especially if you are about to treat yourself to new glasses or contact lenses! However, working out what all those numbers and abbreviations mean can feel like trying to read a foreign language. Don’t worry: it is not as complicated as it looks. In this Lens4Glasses article we will walk you through it step by step, so you understand every part of your eye prescription and know exactly what each term says about your vision.
Key terms and common abbreviations
Before we dive into the numbers, it is essential to get familiar with the abbreviations you will usually see:
- OD (oculus dexter): your right eye. Some prescriptions simply write R.
- OS (oculus sinister): your left eye. It may also appear as L.
- OU (oculus uterque): both eyes (sometimes used for binocular values).
- SPH (sphere): the lens power needed to correct short-sightedness, or myopia (numbers with a minus sign), or long-sightedness, or hyperopia (numbers with a plus sign). It is measured in dioptres.
- CYL (cylinder): the lens power needed to correct astigmatism. It is also measured in dioptres.
- Axis: a number between 0 and 180 degrees that shows the orientation of the astigmatism. It always goes together with the cylinder.
- ADD (addition): the extra power needed for close-up vision when you have presbyopia (age-related long-sightedness). It only appears on prescriptions for multifocal or progressive lenses, or for reading glasses.
- Prism / Base (P / B): less common, it is used to correct eye alignment problems (strabismus or squint). The prism shows the power and the base shows the direction.
Decoding the numbers: step by step
1. Right eye (OD) and left eye (OS)
Your prescription is organised in rows, one for each eye. The first row is usually for the right eye (OD or R) and the second for the left eye (OS or L).
2. Sphere (SPH)
This is the first important number. A minus sign (-) means myopia (difficulty seeing far away), while a plus sign (+) means hyperopia (difficulty seeing close up, or generally blurred vision). The higher the number, whatever the sign, the stronger the correction you need.
| Eye | SPH | CYL | Axis | ADD | Prism |
|---|---|---|---|---|---|
| OD (R) | -2.50 | -1.00 | 180 | +2.00 | |
| OS (L) | -2.75 | -0.50 | 090 | +2.00 |
3. Cylinder (CYL) and axis
If you have astigmatism, you will see values in these columns. The cylinder (CYL) shows how much astigmatism you have, and the axis shows its orientation in degrees. If these columns are empty or say “PL” (plano) or “DS” (dioptre sphere), it means you have no astigmatism, or it is so slight that it does not need correcting.
4. Addition (ADD)
If your prescription includes an “ADD” value, it usually means you have presbyopia and need extra power to see up close. This value is always positive and is usually the same for both eyes.
5. Prism (P / B)
As we mentioned, prism is less common and corrects eye alignment problems. If you have it, your optometrist will give you specific details about the power and direction (base).
A practical example
Let’s imagine a prescription like this one:
| Eye | SPH | CYL | Axis | ADD |
|---|---|---|---|---|
| OD (R) | -3.00 | -1.25 | 175 | +2.50 |
| OS (L) | -2.75 | -0.75 | 005 | +2.50 |
This means:
- Right eye (OD): you have -3.00 dioptres of myopia, -1.25 dioptres of astigmatism with the axis at 175 degrees, and you need +2.50 dioptres to see up close (presbyopia).
- Left eye (OS): you have -2.75 dioptres of myopia, -0.75 dioptres of astigmatism with the axis at 5 degrees, and you also need +2.50 dioptres to see up close.
Why is it important to understand your prescription?
Understanding your prescription puts you in control as a customer. It lets you ask your optician better-informed questions, check that your new glasses or contact lenses have been made correctly and better understand how your eyesight is changing over time. Professionals will always be there to help, but having this knowledge gives you more control over decisions about your eyes.
When to see a professional
Although this article gives you a guide, it is always essential to consult your ophthalmologist or optometrist if you have specific questions about your prescription or notice changes in your vision. They are the only ones qualified to interpret your prescription and recommend the best solution for you.
Conclusion
Reading your eye prescription no longer needs to be a mystery. Once you know the key terms and how the numbers are laid out, you will have a clear picture of your visual needs. This information will be invaluable when you choose your next glasses or contact lenses, and when you order new single vision lenses or progressive lenses for the frame you already own, so that your vision is always at its best.