Your glasses prescription explained
What do the numbers and words mean on your prescription? Watch the video from one of our optometrists, Dr Nigel Best DOptom MCOptom FBCLA FAAO, to find out how to understand your eye prescription:
How to read your prescription
What does SPH mean on my glasses prescription?
The SPH on your glasses prescription stands for sphere. Sphere refers to the power of your lens (measured in dioptre units) to correct the degree of short or long sight you have. Commonly, this number will be different for each eye.
A + in the box indicates that you are long-sighted which means you find it difficult to see things close to you. A – shows that you find it hard to see things that are far away without glasses. The number might be very small, like 0.25, or a large number, like 6.00. The higher the number, the stronger the prescription lenses required. This can influence your choice of frames as the higher the prescription, the greater the curve of the lens needed.
What does CYL mean on my glasses prescription?
The abbreviation CYL on your prescription stands for cylinder. This indicates the amount of lens power required to correct any astigmatism present.
Astigmatism results from having a cornea that is not perfectly spherical. An empty box means there is no astigmatism and your eyes are perfectly spherical. A low number, like 0.25, means your eyes are not quite round and a higher number, like 3.00, means your eyes are quite oval.
What does AXIS mean on my glasses prescription?
The axis number on your prescription tells your optician which direction they must position any cylindrical power in your lenses (required for people with astigmatism).
This number shows the orientation or angle in degrees from 1 to 180. The number 90 means vertical position and 180 horizontal. A higher number for the axis doesn’t mean that your prescription is stronger — it just describes the position of the astigmatism.
What does PRISM mean on my glasses prescription?
If you require a prismatic correction in your glasses this usually indicates that your eyes aren’t working well as a pair. Prism lenses will provide the correction you need and help prevent double vision or headaches.
What does the BASE mean on my glasses prescription?
The base simply tells the lab where to put the prism in your glasses.
How to obtain your prescription
If you have any questions about your prescription or would like a copy, you can contact your local store, or the store where you last had your eye test.
Your local store will then be able to update your details accordingly or provide you with the information you need.
What do ‘+’ and ‘-’ refer to on my prescription?
Put simply, a ‘+’ shows if you are long-sighted and a ‘-’ shows if you are short-sighted.
FAQs
What is pupillary distance (PD)?
Essentially, it’s the distance between the centres of your pupils (the black bits in the middle of the eyes). Your PD is an important part of your glasses prescription that makes sure you’re looking through the best spot in your lenses for clear vision. You’ll need this measurement if you’re buying your specs online — find out how to measure your pupillary distance here.
What does ADD mean on my glasses prescription?
If you’re over 40, you may have a number here. This is your reading addition and relates to the amount of additional correction needed to help you to focus at close distances. If you have a reading addition you will have different prescriptions for distance and reading. Sometimes another add for intermediate prescriptions is also required.
In some cases, the near add is written on the prescription in a different format and is combined with the distance prescription to provide the near prescription.
If you have any questions about your prescription or would like a copy of your prescription, get in touch with your local store, or the store where you had your eye test.
What is a dioptre?
A dioptre is a unit measuring a lens's refractive power (its ability to bend light passing through it).
What is the minimum prescription for glasses?
Your optician will always be able to find the right glasses lens for you, no matter how low your prescription might be. Glasses go up in strength by factors of .25 (1.50, 1.75, 2.00). The weakest glasses lens prescription is typically 0.50 (however your optician should accommodate for lower prescriptions if needed).
How do I get a copy of my eye prescription?
You will be provided with a copy of your prescription following your eye test. If you need a replacement copy for any reason, just ask your optician and they’ll provide one for you.
My prescription from my optician and on my glasses voucher are different, what should I do?
There are two ways of expressing a prescription. There’s a quick way to find out whether you’ve been given the same prescription in two different forms — it’s known as transposition.
To convert one from the other you should do the following:
- SPH + CYL = SPH of the other prescription
- Both CYLs should be of the same value but one is negative while the other is positive
- For AXIS, add or subtract 90 degrees to the original axis value to obtain the new value which must be between 0 and 180, for example if the original is 30 you add 90 to get 120; if the original is 140 you subtract 90 to get 50.
If the results of your conversion don’t match, your optician will be able to tell you which of the two prescriptions to use.
