Your prescription is a set of measurements describing what each eye needs to focus clearly. Once you know what the columns mean, it stops looking like code.
Here is each part, in the order it usually appears.
OD and OS
OD is your right eye. OS is your left eye. They come from the Latin oculus dexter and oculus sinister.
You may also see OU, which means both eyes together.
Some prescriptions simply say Right and Left. Same thing.
Sphere (SPH)
The main correction, measured in diopters.
A minus sign means nearsighted. Distance is blurry, near is usually clearer. A minus number is the most common thing we write.
A plus sign means farsighted. Your eye has to work harder to focus, particularly up close.
The further the number is from zero in either direction, the stronger the lens. A larger number does not mean your eyes are unhealthy. It describes shape and focus, not health. Eye health is a separate part of the exam entirely, which is why the exam includes more than a refraction. See what happens during a comprehensive eye exam.
Cylinder (CYL) and Axis
These two always travel together, and they describe astigmatism.
Cylinder is how much astigmatism correction you need. Astigmatism means the front of the eye is shaped a little more like a football than a basketball, so light focuses unevenly.
Axis is a number from 1 to 180 describing the orientation of that correction. It is an angle, not a strength. An axis of 180 is not stronger than an axis of 10.
If these boxes are blank, you have little or no astigmatism. Astigmatism is extremely common and is not a disease. More in what is astigmatism, and does it get worse.
Add
Extra magnifying power for near vision, written as a plus number.
This appears once near focus starts changing, usually somewhere in the forties, as the lens inside the eye becomes less flexible. It is what drives reading glasses, bifocals, and progressive lenses. See why you suddenly need reading glasses in your 40s.
The add is usually the same for both eyes.
Prism and base
Most prescriptions do not have these. When present, prism helps the eyes align comfortably as a pair, and base describes the direction.
What is not on your prescription
This is where most confusion starts.
Your glasses prescription does not include the information needed for contact lenses. A contact sits directly on the eye rather than twelve millimeters in front of it, which changes the power required, and a contact-lens prescription also specifies brand, base curve, and diameter. That comes from a fitting. See regular eye exam vs. contact-lens exam.
It also does not include your pupillary distance or the other measurements that position the lens correctly in your chosen frame. We take those in the optical using iTerminal 2, which captures five measurements rather than relying on pupillary distance alone. More in personalized eyewear and lenses.
Common questions
Is minus 4 bad?
It is a moderate amount of nearsightedness and says nothing about the health of your eyes. Plenty of people wear it comfortably.
Why did my numbers change?
Prescriptions shift over time, and more quickly in childhood and the teens. A sudden change is worth investigating, since blood sugar changes can shift a prescription temporarily. See medical eye care.
My two eyes have very different numbers. Is that a problem?
It is common. It can affect lens thickness and how the lenses are made, which our optical team accounts for.
Can I just order online with this?
You can order glasses with a valid glasses prescription, though the fit and centration measurements matter more than people expect. You cannot order contacts with it.
How long is my prescription valid?
Ask us, since it depends on your age and eyes. When it expires, that is a prompt for an exam rather than paperwork.
Bring it in and we will walk you through it
If you want yours explained line by line, ask at your next visit. We would rather you understand what we are recommending than just the numbers.
Request an appointment or call 805-925-9575.
