Holding your glasses prescription, you must be wondering what the numbers mean. Your prescription consists of a mixture of different numbers and letters and can be slightly harder to understand if it’s your first time prescription or you’ve never really given it much thought.
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.
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
ADD and Prism
“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).
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.
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