You leave the exam with a slip of paper covered in Latin-looking abbreviations and a grid of small numbers. Nobody explains it, because to the staff it is as ordinary as a shopping list.
It is not complicated once you know what each column is for. Every entry does one of three jobs: it says which eye, it says how much correction, or it describes how the lens should sit in front of your face.
Here it is, column by column.
OD, OS and OU: which eye is which
These come from Latin. OD is the right eye, OS is the left eye, and OU means both eyes together. On most printed forms the right eye sits on the top row.
Some practices have dropped the Latin and simply print Right and Left, which is clearer and just as valid. One thing to watch: the same two letters, OD, also stand for Doctor of Optometry after a person's name. On a prescription grid it means the right eye. After a name on a door, it is a credential.
SPH, CYL and AXIS: the core of the correction
These three columns carry the actual lens power, all measured in diopters, usually in quarter steps.
SPH (sphere)
This is the main focusing power, and the sign in front of it tells you the direction. A minus number corrects nearsightedness, where distance is blurry. A plus number corrects farsightedness. If you see PL or the word plano, it means no spherical power is needed in that eye. The further from zero, the stronger the lens.
CYL (cylinder)
This column appears when you have astigmatism, meaning the eye focuses light unevenly across different directions. The number is the extra power needed to even that out. A blank or a dash means none was recorded. Offices differ in whether they write cylinder in plus form or minus form, so the same correction can look like two different sets of numbers on two different slips.
AXIS
Axis is not a strength. It is an orientation, written as a whole number from 1 to 180 degrees, describing where the cylinder correction has to be lined up. It only ever appears alongside a cylinder value, and a cylinder value is incomplete without it.
ADD, prism and base
ADD is extra plus power for close work, added into the lower part of a bifocal or progressive lens. It usually shows the same figure for both eyes, and it becomes common from the mid-forties onward as near focusing gets stiffer.
Prism shows up on a minority of prescriptions. It shifts the image slightly to help two eyes that are not lining up on the same point, and it is measured in prism diopters. It always comes with a base direction: base up, base down, base in toward the nose, or base out toward the ear, often shortened to BU, BD, BI and BO.
PD and the fitting measurements
Pupillary distance, or PD, is the space between the centers of your pupils in millimeters. It is not a correction. It tells whoever makes the glasses where to place the optical center of each lens so you are looking through the right part of it.
PD can be written as a single total, or as two separate numbers when each eye is measured from the bridge of the nose. Progressive and bifocal lenses also need a segment height, which depends entirely on the frame you pick and the way it sits on your face, so it is measured after you choose.
You may also see shorthand for lens style and extras: SV for single vision, BF for bifocal, PAL for a progressive lens, AR for an anti-reflective coating, plus notes for photochromic or polarized lenses. Those describe the product, not your eyes, and they are usually priced separately. It is fair to ask which of them are included before you agree to anything.
Contact lenses need their own prescription
A glasses prescription cannot be used to order contact lenses, and the reason is physical. A contact lens sits on the eye rather than a centimeter or so in front of it, so the power often differs, and the lens has to match the shape of your cornea.
That is why a contact lens prescription carries extra entries: BC for base curve, DIA for diameter, and a named lens material and design, because these numbers are only valid for that specific lens. It also requires a fitting and a check that the lens behaves well on your eye. Contact lens prescriptions expire, and so do glasses prescriptions in most states.
What the numbers do not tell you
A prescription describes how light needs bending. It says nothing about the health of your eyes. A large number is not a disease, and a small one is not a clean bill of health, which is why the exam includes far more than a refraction. If you want a sense of what else gets checked, the list of common services and conditions covers the usual ground.
These numbers also do not rank people against each other. Someone with a stronger correction is not seeing worse than you once both of you have the right lenses in front of your eyes.
And if a new slip looks nothing like your old one, resist the urge to work out the difference yourself. Between rounding, plus and minus cylinder conventions, and genuine change over a couple of years, the arithmetic is easy to get wrong. Ask the office to compare them.
Common questions
Why is my PD missing from my prescription?
It is a fitting measurement rather than part of the correction, so not every office prints it. You can ask for it, and most will give it to you.
My axis changed by a few degrees. Is something wrong?
Small shifts in cylinder and axis between exams are common and often just reflect measurement on the day. Ask about it at your visit rather than worrying between appointments.
Can I keep using an expired prescription?
An optical shop will generally not fill one. With contacts especially, the expiry exists so someone checks how your eyes are handling the lenses. Book the exam instead.
Keep your prescriptions in one place, because a few years of them side by side tell a story that any single slip cannot. When the current one runs out, or when something on it stops making sense, that is the moment to search for a local eye doctor who accepts your vision benefits and have the numbers explained to you in person.