Glasses prescription explained
Aug 02, 2024 in Eye Tests
Getting regular eye check ups are important for ensuring your eyes are healthy and you have the correct prescription. During an eye test, you’ll have your vision tested and are likely to have your eye health tested as well through a variety of tests. You may be recommended to wear a pair of glasses at the end of your session with your optometrist or, if you already wear glasses, might receive an updated prescription.
A glasses prescription is made up of a combination of different letters and numbers. This information describes the type and strength of glasses or lenses you might need. Glasses and contact lenses often have different prescriptions – so if you already wear glasses, you’ll need to have another test if you’re thinking about wearing contact lenses. If you’re looking at your prescription and wondering what all the different numbers and symbols mean, read on to learn more about glasses prescription and to have your eye prescription explained.
How to read your eye prescription
If you’re new to wearing glasses or a regular wearer of contact lenses, you might be wondering what all those letters and numbers mean on your glasses prescription. It’s likely that your contact lens prescription is different to your glasses prescription, which is another reason why being able to read them is important.
In order to read your prescription, you first need to understand what the various abbreviations and letters mean.
What do OD and OS mean?
When looking at your prescription, you may see the terms OD and OS. They explain which eye the prescription is for and are abbreviations for the Latin terms for your eyes:
- OD: stands for oculus dexter, or your right eye.
- OS: stands for oculus sinister, or your left eye
- OU or ODS: stands for oculus uterque/oculus dexter et sinister, or both eyes. OD refers to your right eye and OS refers to your left eye, as they often have different needs. When looking at the prescription, the left and right eyes are reversed to match your optometrist’s face-on view of your eyes.
What does SPH mean?
SPH stands for sphere and is the amount of lens power you need for clear vision. It describes how long or short-sighted you are and is followed by a number, which is the unit used to measure the correction (dioptres). Usually, the further the number is from 0, the stronger the correction you need.
If you see a minus sign (-) before the numbers, this means you are short-sighted (myopic) – you can see things up close but objects further away can appear blurry. The higher the number after the minus, the more myopic you are.
Plus signs (+) mean you are long-sighted (hyperopic). You are likely to find seeing far away things easier than close up. The more long-sighted you are, the higher the number following the plus sign will be.
What does CYL mean?
Standing for cylinder, CYL explains the amount of lens power needed to correct astigmatism – a condition where the eye doesn’t evenly focus. The CYL can be negative or positive. The higher the number, the stronger the astigmatism. If you don’t have astigmatism, or the effect is too small to need correction, you won’t have a CYL number on your prescription.
What does AXIS mean?
AXIS and CYL work together to correct astigmatism. AXIS describes the orientation of the cylinder (astigmatic lens) required to correct your astigmatism. It’s displayed as a positive number between 0 and 180 degrees and acts almost like a map of your eye.
What does BASE mean?
Some people have difficulty in eye coordination and require a special prismatic lens to help their eyes work together. The prismatic lens alters the direction of light travel without altering the focus. Prism stands for the amount of prism (in prism diopters) required, while base describes the orientation required for the prism to aid the eyes work together.
What does ADD mean?
ADD describes the added lens power needed to make it easier for you to see at a closer range such as reading or using a computer. You will see this on a prescription for reading glasses or for the bottom part of varifocal lenses. The ADD number is usually the same for both eyes somewhere between +0.75 and +3.00 D.
What does BVD mean?
BVD stands for back vertex distance. For higher prescriptions altering the position of the lens will alter its effect. In these cases the distance between the lens and surface of the eye is measured. This can be used to adapt the prescription should the desired spectacles sit at a different distance from the eye, maintaining equivalent focus.
How do I get a copy of my eye prescription?
You will be provided with a copy of your prescription after your eye test. This is for your reference and records but if you need a replacement copy for any reason, simply ask your optician and they’ll provide you one.