Understanding your prescription
You’ve just had an eye exam, and your optometrist or ophthalmologist has given you a new glasses prescription, full of all the measurements you need to attain clear and comfortable vision.
They may have told you you’re short or long-sighted, or that you have astigmatism, but when you look at your prescription, it doesn’t simply say “long-sighted”. Rather, there are an array of abbreviations with decimal numbers, pluses and minuses, and strange symbols underneath.
Little did you know, you need to be an eye expert and speak a little Latin to be able to read it.
On your prescription, you may see OS (oculus sinister) which means left eye, and OD (oculus dextrus) which means the right. You may also see OU, which indicates something involving both eyes. But other than that, thankfully the rest is at least in English. But you’ve still got the task of figuring out the meaning of strange headings such as SPH, CYL, Axis, Prism, Base, and ADD.
Your eye doctor may have offered you help in understanding your prescription. In essence explaining that, in general, the further away from zero the numbers are, the poorer your vision and the more likely you are to need vision correction. But if you’re anything like me, you will have already forgotten what they told you. Let’s recap and find out exactly what each of the numbers and words mean.
Sphere (SPH)
Sphere indicates the amount of additional lens power you need to correct short or long-sightedness. The term ‘sphere’ refers to the fact that the correction is “spherical”, or equal across the entire eye.
The number you’ll see here is in diopters – the unit of measure for the refractive or light-bending power of the lens. If you are short-sighted, the number will have a minus (-) sign before it; if you are long-sighted, the number will have either a plus (+) sign before it or not be preceded a symbol.
Cylinder (CYL)
Rather than spherical correction that works across all meridians of the eye, ‘cylinder’ corrects for an irregularly shaped cornea – otherwise known as astigmatism.
The number here represents the lens power you need for astigmatism. If the box is empty, it means there’s no or not enough astigmatism present to necessitate correction. Cylinder follows the general rule of the further the number is away from zero, the more lens power is needed.
You may also see that the number is preceded by a plus or a minus sign. This shows if you need correction for short or long-sighted astigmatism.
Axis
The axis describes the direction of astigmatism, measured in degrees.
With a number from 1 to 180, 90 corresponding to the vertical meridian of the eye and 180 to the horizontal (imagine a protractor placed on the surface of the eye), you are given a number that defines at what angle your lenses should be positioned in the frame.
Prism and Base
Present in only a small number of prescriptions, the ‘prism’ measurement describes how much prismatic power you need to compensate for eye alignment problems.
This normally means your eyes don’t work properly as a pair, often due to a muscular imbalance between the eyes. The base indicates the direction of the prism, whether up, down, towards your nose, or towards your ears, with four abbreviations: BD (base down), BU (base up), BI (base in), and BO (base out).
ADD
If you are in need of reading glasses, you may see ADD on your prescription. This is the added magnifying power needed to focus on objects at close distances. Depending on the severity of the ADD value, ranging from +0.75 to +3.00 D, you may need bifocal or varifocal lenses.
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