How to understand glasses prescription
We help decode all those words and numbers
If you’ve never had to read a glasses prescription before, trying to decode yours might bring back bad memories of high school algebra. But all those words and numbers aren’t that difficult to understand once you know what they mean. We spoke to Dr Joe Paul, Head of Professional Services Optometry at Specsavers, to have your eye prescription explained.
What do the words on my eye prescription mean?
Your glasses prescription will generally contain a table with two rows labelled OD and OS and six columns labelled SPH, CYL, AXIS, ADD, PRISM and BASE. Sometimes, PRISM and BASE are combined in the same column. Here’s what these words and abbreviations mean:
OD: oculus dexter is the Latin term for right eye
OS: oculus sinister is Latin for left eye
SPH (sphere): This is the amount of lens power measured in dioptres. A “+” in front of the number means you’re long-sighted (also called far-sightedness or hyperopia) and you have trouble seeing objects up close. A “-” indicates you’re short-sighted (also referred to as near-sightedness or myopia) and you can’t see distant objects clearly
CYL (cylinder): Cylinder is the amount of lens power required for astigmatism – a condition characterised by an irregularly shaped cornea that causes blurred vision when looking at both near and far objects. If this box is empty, your eyes are perfectly spherical and you don’t have astigmatism
AXIS: If there’s a number in the cylinder column, there will also be one in the axis column. Axis indicates the orientation of your astigmatism measured in degrees
ADD: Add refers to the additional power needed for close-up work, such as reading. If you have a number in this box, your glasses will be equipped with extra correction to help you focus at close distances. The number is generally the same for both eyes
PRISM: This indicates eye-alignment issues, which means your eyes don’t work well together and you may experience double vision. The specified number will correct these issues
BASE: Base refers to the position of the thickest edge of the prism. It can be at the inner edge of the lens (Base In or BI), the outer edge (Base Out or BO), the top (Base Up or BU) or the bottom (Base Down or BD)
What are the numbers on my glasses prescription?
In each of the categories on your eye prescription, a small number indicates a weak prescription and a high number means you have a strong prescription – with the one exception being axis. “A higher number for the axis does not mean that your prescription is stronger,” says Joe. “It simply describes the position of the astigmatism.”
If your sphere column reads between -0.25 and -2.00, you have mild short-sightedness. If it’s below -5.00, you have high short-sightedness. The same goes for positive numbers: +0.25 to +2.00 is mild long-sightedness and over +5.00 is high long-sightedness. “It is common for the number to be different for each eye,” says Joe.
A cylinder of 0.25 means your eyes aren’t exactly round, while a cylinder of 3.00 means your eyes are more oval-shaped. “The number describes the difference in dioptres between your cornea's steepest and shallowest curves,” explains Joe.
Do I need to know my glasses prescription?
The Optometry Board of Australia’s Guidelines for the Prescription of Optical Appliances indicates that your optometrist must provide you with a copy of your prescription if you request it at the end of the consultation. You can keep this somewhere safe for future
reference next time your glasses need an update (although if your vision has changed since then, you may get a new prescription).
Are glasses and contact lenses prescriptions the same?
“As a contact lens sits directly on the eye's surface, often glasses and contact lens prescriptions are not the same,” says Joe.
If you choose to make the switch to a different type of eyewear, you’ll need to see your optometrist again for a new prescription.
Related: Glasses or contact lenses: what you need to consider
When to get your eyes tested
“Optometrists recommend you have your eyes tested at least every two years unless otherwise directed,” says Joe.
Related: What happens in an eye test?
Ready to book an appointment? Check out the nib First Choice Optical network to search for local optometrists. First Choice is our community of specially selected health providers who are committed to delivering quality care and value for money.
At nib, we offer a range of Extras cover that include optical benefits. If you’re already an nib member, you can check your current policy using Member Services. Alternatively, you can get a quote online in just minutes.
Please note: The tips throughout this article serve as broad information and should not replace any advice you have been given by your medical practitioner.
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In partnership with
Dr Joe Paul
Dr Joe Paul is the Head of Professional Services at Specsavers, providing clinical and professional support to Specsavers optometrists across Australia and New Zealand. Joe has a PhD in glaucoma research and has previously worked as a postdoctoral researcher at the Centre for Eye Research Australia, and as an optometrist at Specsavers and private practice over the last decade.