How to Read an Eye Exam Prescription
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If you’ve had an eye exam and have been handed a prescription for glasses or contact lenses, you might be wondering what all those letters and numbers mean.
If you’re in one of our stores, or you’re able to pop by, then our eye doctors can explain all. But if you want to understand more about your eye health here and now, then check out our glossary of terms and abbreviations below.
If there’s anything you still don’t understand, our eye doctors will be happy to help you.
Read an eye exam prescription – terms and abbreviations
If you’re looking at your eye exam prescription, it’s likely that you’ll see some or all of the letters below, along with + or – numbers. Here’s what it all means:
Source: Dpbsmith via Wikimedia Commons
OD stands for oculus dexter, which is the Latin term for the right eye. So all the numbers and letters corresponding to the ‘OD’ row or column are related to your right eye.
OS stands for oculus sinister, which is the Latin term for the left eye. Remember your eyes won’t necessarily be the same, and in fact, one eye can have a significantly different vision from the other. So the numbers and letters corresponding to the ‘OS’ row or column may not match your right eye.
RE stands for right eye. You might see this instead of ‘OD’, because some optometrists have stopped using the Latin names.
LE stands for, you guessed it, left eye. You’ll see this instead of ‘OS’.
OU stands for oculus uterque, which is the Latin term for both eyes.
SPH stands for the sphere. This indicates the lens power needed to correct either short-sightedness or long-sightedness. If there is a + in front of the number, it means you’re long-sighted, and if there’s a – sign in front of the number, it means you’re short-sighted. The higher the number, the worse your vision is, and the greater amount of lens power needed to correct your vision. Sphere power is measured in diopters.
Diopters is the unit of measurement used to indicate how strong a lens must be in order to correct a person’s vision. Diopters are used to measure sphere, cylinder and add power.
CYL stands for cylinder. This indicates the lens power needed to correct astigmatism. Astigmatism is when the cornea or lens has an abnormal shape. Instead of being perfectly round, it is more elongated, like an American football.
This will mean that your long or short-sightedness won’t be the same in all directions. To correct this, your lens will need a cylinder at the point where your eyeball curves. The + or – in front of the number signifies whether you will have a positive convex lens or a negative convex lens. The higher the number, the worse your astigmatism is.
If you don’t have astigmatism, this section might be blank or left off your prescription entirely. Or you might just have astigmatism in one eye.
Axis determines the location of astigmatism, or in other words, where the curvature of your eyeball is. It will be a number between one and 180, and this expresses the degree to which the cylinder must be placed on the lens. If you have astigmatism, both the cylinder and the axis will be included on your prescription, as the two are closely linked.
If you don’t have astigmatism, this will either be left blank or won’t appear on your prescription.
Add indicates the magnifying power needed to correct presbyopia, which is when the eyes become less flexible with age, making it more difficult to focus on close-up objects. If you’re getting multi-focal lenses, this will be the bottom half of the lens.
Prism lenses are needed if the eyes are not working together, or have an abnormal alignment (meaning they’re not in the same position). This might result in problems such as double or blurred vision, or the inability to see in 3D when using one eye. Prism glasses are also used to help people with hemianopia, which is when someone loses sight in half the visual fields of their eyes. This is typically caused by neurological conditions, such as a stroke.
If you need prism lenses, your prescription will have four more abbreviations, and together they confirm the direction the prism needs to be placed in. They are:
- BU which stands for base up
- BD which stands for base down
- BI which stands for base in
- BO which stands for base out
PD stands for pupillary distance. It’s the distance from the centre of one pupil to the centre of the other. You’ll need this to ensure your glasses are fitted correctly so that you are looking for the centre of each lens. Pupillary distance is measured in millimetres.
BC stands for the base curve. This indicates the curvature on the front side of the lens. The extent of the base curve doesn’t really have anything to do with your prescription. Rather, it’s what an optometry expert believes will deliver the best possible performance. Therefore if you get a new pair of glasses but your prescription remains the same, you may have a different base curve. This won’t bother some people who can readily adapt. But others will have become used to the base curve of their previous glasses and may find it uncomfortable.
DIA stands for diameter. This indicates the width of your glasses or contact lenses. The diameter is not so important when it comes to glasses, as it’s more a matter of personal preference. But it’s vital to get the diameter specific to your eyes when choosing contact lenses, otherwise, they won’t fit.
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Or if you already use glasses or contact lenses, we can make sure your current prescription still matches your requirements and will advise you if it needs to be updated.
If you do need a prescription and you’d like to understand more about what it means for you and your eye health, our eye doctors will be more than happy to share their expert knowledge with you.