After your eye test, your optometrist will give you a written prescription if you need glasses or a change to your existing pair. This prescription uses standardised abbreviations that can look confusing at first glance, but each term has a straightforward meaning. Understanding your prescription empowers you to shop confidently — whether you buy from your local optician, an online retailer, or a different provider.
The key terms on your prescription are: OD (or R) for your right eye and OS (or L) for your left eye. SPH (sphere) indicates the lens power needed to correct short-sightedness (a minus sign, e.g. -2.00) or long-sightedness (a plus sign, e.g. +1.50). CYL (cylinder) and Axis are used together to correct astigmatism — CYL measures the degree of astigmatism, and Axis (a number between 1 and 180) indicates its orientation. If these boxes are blank, you have no significant astigmatism. Add (addition) is the extra magnifying power needed for reading or close work, typically prescribed for people over 40 with presbyopia. Prism and Base are less common and are used to correct double vision or eye alignment problems.
Your prescription will also include a date and an expiry period. In the UK, a glasses prescription is typically valid for two years (one year for children under 16), though your optometrist may specify a shorter validity if your eyes are changing rapidly. You are legally entitled to a copy of your prescription after your eye test, free of charge, and you are not obliged to buy your glasses from the practice that tested your eyes — you are free to take your prescription to any optician or online retailer.
It is worth noting that a glasses prescription and a contact lens prescription (known as a specification) are not the same. Contact lenses sit directly on the eye rather than in front of it, so the powers may differ, and additional measurements such as base curve and diameter are required. You need a separate contact lens fitting to obtain a contact lens specification, and this must be issued by the practitioner who fitted your lenses.
If any part of your prescription is unclear, ask your optometrist to explain it. They are happy to help, and understanding your prescription means you can make better-informed choices about your eyewear. Keep a copy of your prescription in a safe place — it is useful if you need to order replacement glasses quickly, for example while travelling.