Understanding your eye prescription

When you have an eye exam, your eyes are tested in a number of different ways. The resulting measurements are recorded in your eye prescription. If you’re looking at your prescription and trying to decode what it all means, we’re here to help! Discover the meaning behind those mysterious words and abbreviations in our guide to understanding your eye prescription.

Sphere

The Sphere column in your prescription (sometimes abbreviated to SPH) refers to spherical correction – the lens power needed to correct your vision across the curve of your eye, which should be the same all the way around if you don’t have astigmatism.

The units for this part of the prescription are dioptres, or D for short, and usually the numbers range from -6.00D (very short-sighted) to +6.00D (very long-sighted).

Cylinder (CYL)

Sometimes, people’s eyes are irregularly shaped. This is called astigmatism. If you have this condition, your ophthalmologist will record a cylinder measurement (sometimes noted down with the abbreviation CYL). This will have a plus or minus sign in front of it, depending on whether you are also long- or short-sighted. You will need special cylindrical lenses in order to correct your vision.

eye test

Axis

Axis is another metric relating to astigmatism, so this part won’t be filled in for every prescription. For those with the condition, the axis column describes the angle or direction of your astigmatism. Your eyes are shaped like a rugby ball rather than a regular sphere, so there’s a position on your cornea where your cylindrical lens should be placed in order to provide vision correction. The axis measurement describes this spot in degrees, so it will be a number between 1 and 180.

ADD

Will you need reading glasses or bifocals in addition to your regular specs? The ADD part of your prescription will tell you this. ‘ADD’ simply refers to any ‘added’ lens power that you may need in order to focus on close objects, such as when you read a book or look at a computer screen. The numbers here range from +0.75 to +3.00D.

Prism and Base

In rare cases, the right eye and the left eye don’t work well together, which can lead to problems focusing. If you have a number in the Prism column of your eye prescription, it means you need special prism lenses which bend light to correct the problem. The prism base is the direction in which the prism will need to be aligned.

Abbreviations

Even with an understanding of the basics, prescriptions can be hard to follow if they include abbreviations. Optometrists and ophthalmologists sometimes use abbreviations such as:

OD: short for Oculus Dextrus. This is Latin for “Right eye”.
OS: short for Oculus Sinistrus. This is Latin for “Left eye”.
OU: short for Oculus Uterque. This is Latin for “Both eyes”.
DV: short for Distance Vision.
NV: short for Near Vision. It’s sometimes used instead of ADD.

Contact lens prescriptions

Even though they are designed to correct the same visual problems, contact lens prescriptions vary slightly from glasses prescriptions. That’s partly because of the way they are worn: while spectacles are positioned some distance away from the eye, contacts cover the surface up close, so they’ll need to be designed differently in order to help you see. Contact lens prescriptions also require a little more information, such as measurements of the curve and diameter of your eye.