How to Read an
Eye Prescription

 

How to Read Your Eye Prescription

Your eye prescription will explain whether you are:

• Short-sighted,
• Long-sighted,
• Have astigmatism.

This is a type of medical shorthand using a combination of abbreviations and numbers. You need to know what these mean if you want to know how to understand your eye prescription.

 

OD, OS and OU or ODS

The first abbreviations explain which eye the prescription is for. They are abbreviations for 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.

Sphere (SPH)

SPH or sphere is the amount of lens power you need to see clearly. It measures how long or short-sighted you are. It is followed by a number which is the unit used to measure the correction (dioptres). Generally, the further the number is from 0, the stronger the correction you need.

If the SPH number has a minus sign (–), it means that you are short-sighted. Short-sightedness (myopia) means you can clearly see objects that are close. Objects that are further away will be difficult to see and blurry. The more short-sighted you are, the more minus the number will be.

If the SPH number has a plus sign (+), it means that you are long-sighted. With long-sightedness (hyperopia) you will find it easier to see objects that are further away than objects that are close up. The more long-sighted you are, the more plus the number will be.

Cylinder (C or CYL) and Axis

Astigmatism is a very common condition where your eyeball isn’t completely round. When an eyeball is completely round, the light entering your eye bends evenly forming a clear image on the back of the eye. For people with astigmatism the cornea is irregular, so its curvature is not spherical but toric (like the side of a ring doughnut). Light rays coming from objects in the field of vision will not “land” at a single point, which means that vision will be distorted and more out of focus in some directions than others.

If you have astigmatism, your prescription will describe how strong it is and where the irregular curve is on your eye. This is described in three parts:

• Sphere (SPH): as explained above, this is whether you are long or short-sighted and by how much. 
• Cylinder (CYL): This explains the amount of lens power needed to correct your astigmatism. It 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.
• Axis: This explains where the irregular curve that is affecting your vision is. It is a positive number between 0 and 180 degrees that acts like a map of your eye.

CYL and Axis work together to correct your astigmatism. If you don’t have astigmatism in one or both of your eyes, you will only have the SPH measurement on your prescription. If you have astigmatism you will have SPH, CYL and AXIS measurements.

ADD, Prism and Pupillary Distance (PD)

“Add” is the added lens power needed to make it easier to read. You will see this on prescription for reading glasses or for the bottom part of varifocal lenses. These help to correct natural age-related long-sightedness (presbyopia). It’s usually the same for both eyes and between +0.75 and +3.00 D.

Prism is the amount of prismatic power needed if you have eye alignment problems. This is measured in prism dioptres (p.d. or a triangle). It also explains where to position the prism on the glasses using BU = base up; BD = base down; BI = base in (towards nose); BO = base out (towards the ear). Not all prescriptions will require a prism correction.

Pupillary Distance (PD) is an important part of your prescription as it explains where the optical centre of your lenses should be to give you the best vision. It measures the distance between your two pupils (the black circles in your eyes). A PD is important for fitting your prescription to the spectacle frame.

Contact lens prescription

How to read a contact lens prescription

Your contact lens prescription may not be the same as your glasses. This is to do with the distance of the lenses from your eyes. Glasses are designed to sit on your nose, a few millimetres from your eyes. Contact lenses are designed to fit exactly on the surface of your eye.

A contact lens prescription consists of:

• Prescription: It will always have a spherical number, and will sometimes have a correction for astigmatism,
• Base curve: This describes the curvature of the contact lens. It’s usually between 8 and 10.
• Diameter: The size of the lens.

It will also describe the manufacturer, brand and expiry of your lenses specification.

If you still have questions on how to read a glasses prescription or a contact lens prescription, please contact your optometrist.

Other notes

Your prescription may also describe other features of your glasses or contacts such as photochromic lenses (get lighter or darker depending on the light), scratch resistant coating, or blue light or anti-reflective coatings for screen work.

 

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