A glasses prescription is a short table of numbers that tells a lab how to make lenses that correct your eyesight. It is written for the lab rather than for you, which is why it can look like a cipher, but every value on it means one plain thing.
Here is a typical prescription, with the example used throughout this guide:
| SPH | CYL | Axis | ADD | Prism | |
|---|---|---|---|---|---|
| OD (right) | -2.25 | -0.75 | 180 | ||
| OS (left) | -2.00 | -0.50 | 170 |
PD: 63
Each column has a one-sentence job. SPH is how strong the lens needs to be. CYL and axis correct astigmatism. ADD is for reading. PD tells the lab where to centre the lens over each eye. The rest of this guide walks through the table one column at a time and ends with the five errors that most often lead to lenses being made wrong.
OD and OS: which eye is which
OD stands for oculus dexter, Latin for right eye. OS stands for oculus sinister, left eye. Some prescriptions use OU, oculus uterque, meaning both eyes, when the values are the same.
Many optometrists have moved to plain R and L, or Right and Left. They mean exactly the same thing.
The convention that trips people up: the right eye is always listed first, on the top line. Your right eye is the one on the right side of your own face, not the right side of the page as someone looks at you. When you copy values into an order form, keep that straight. Swapping OD and OS is one of the most common errors in online lens orders, and it produces a pair of glasses that feels subtly wrong without an obvious reason.
SPH: the main strength of the lens
SPH stands for sphere. It is the main corrective power of the lens, measured in dioptres (D), and it tells you whether you are short-sighted or long-sighted and by how much.
- A minus sign (−) means you are short-sighted (myopic). Distant things are blurred. The lens is thinner at the centre and thicker at the edge.
- A plus sign (+) means you are long-sighted (hyperopic). Near things are harder to focus on. The lens is thicker at the centre and thinner at the edge.
- PL, Plano, 0.00 or ∞ means no spherical correction is needed in that eye.
The further the number is from zero, the stronger the correction. A −1.00 is a mild prescription. A −5.00 is a strong one. The sign matters as much as the number: a +2.00 and a −2.00 are completely different lenses.
Values move in steps of 0.25. If your prescription reads −2.25, the next possible values are −2.00 and −2.50. Anything finer than that is not a prescription value.
In the example above, the right eye is −2.25 and the left is −2.00. Both are short-sighted, with the right eye slightly more so.
CYL and axis: astigmatism
Astigmatism means the front of the eye is curved more steeply in one direction than another, like the back of a spoon rather than a ball. It makes light focus at two points rather than one, so things are blurred or stretched in one orientation. It is very common and it is corrected with two values that always travel together.
CYL stands for cylinder. It is the extra power needed to correct the astigmatism, also in dioptres, and it is usually written with a minus sign in the US and Canada. A blank, DS, SPH or 0.00 in this column means no astigmatism in that eye.
Axis is the orientation of that correction, in degrees from 1 to 180. It has no plus or minus. It is a direction, not a strength: 90 is vertical, 180 is horizontal, and everything else is in between.
Two rules:
- If there is a CYL, there must be an axis, and vice versa. One without the other is an incomplete prescription.
- The axis is only meaningful with its own CYL. An axis of 180 with CYL −0.75 and an axis of 180 with CYL −2.00 are different corrections.
In the example, the right eye has CYL −0.75 at axis 180 and the left has CYL −0.50 at axis 170. Both are mild astigmatism.
Why the axis has to be exact
A sphere value is a round number of power that can be off by a step and still feel roughly right. An axis is a direction, and getting it wrong rotates the correction. A 10-degree error on a low CYL is barely noticeable. On a higher CYL it produces a visible tilt in straight lines and a headache by lunchtime. Copy the axis exactly as written, and if the handwriting is ambiguous, call the practice.
Plus-cylinder prescriptions
Some ophthalmologists write CYL as a positive number instead of a negative one. It describes the same correction in a different notation, and a lab converts between the two routinely. If your CYL values are positive, you do not need to change anything yourself. Enter them as written and the lab transposes them.
ADD: reading power
ADD stands for addition. It is extra magnifying power, always a plus value, added to the bottom part of a lens so you can focus up close. It is the correction for presbyopia, the natural stiffening of the lens inside the eye that makes reading harder from the mid-forties onwards.
ADD values usually run from +0.75 to +3.00, and they are almost always the same for both eyes. If a prescription has an ADD, it has been written for bifocal or progressive lenses, which combine distance and near correction in one lens. A prescription for single vision distance glasses has no ADD.
If you are ordering single vision lenses for distance, you ignore the ADD column. The lab uses only SPH, CYL and axis. The ADD exists on your prescription so that an optician can make you reading or progressive lenses if you want them; it does not alter your distance correction.
If you are ordering single vision reading glasses, the arithmetic is different. The lab adds the ADD to the SPH for each eye, and the result becomes the new SPH. For the example above, with an ADD of +2.00, the reading SPH would be −0.25 for the right eye and 0.00 for the left. You do not need to do this yourself when ordering. Choose reading as the lens type, enter the prescription exactly as written, and the lab calculates it.
At present, Elliot Vaughn prescription lenses are made as single vision for distance. ADD does not affect those orders.
Prism and base: rarely present
Prism is a correction for eyes that do not align properly with each other, which can cause double vision or strain. It is measured in prism dioptres and comes with a base direction: up, down, in or out. Most prescriptions have nothing in this column.
If yours does, it is a specialist correction that online lens ordering generally cannot accommodate. Take the prescription to an optician for lenses to be made in person.
PD: where the lens is centred
PD is your pupillary distance, the distance between the centres of your pupils in millimetres. It tells the lab where to place the optical centre of each lens so you look through the point where the correction is right.
It is sometimes written as one number, such as 63, or as two monocular values, such as 32 / 31. It is also sometimes missing altogether, because not every optometrist writes it down.
Everything about finding, measuring and entering your PD is covered in How to find my PD, including how to measure it with a ruler, how to have someone measure it for you, and what to enter if you have two numbers rather than one.
Other things you may see
Expiry date. In the US, glasses prescriptions are typically valid for one to two years depending on the state. An expired prescription cannot be filled, and more importantly your eyes may have changed since it was written.
BVD or vertex distance. The distance from the back of the lens to the front of the eye, usually around 12 mm. Only relevant on strong prescriptions, and the lab assumes a standard value if it is not given.
Contact lens values. A contact lens prescription is not a glasses prescription. It has different powers, plus base curve and diameter, because the lens sits on the eye rather than in front of it. It cannot be used to order glasses.
DV and NV. Distance vision and near vision. Some prescriptions list them as two separate rows rather than using an ADD column. For distance glasses, use the DV row.
The five errors that get lenses made wrong
These are the mistakes that come up again and again in online lens orders. Checking for them takes a minute.
- Swapping right and left. OD is right, OS is left, and the right eye is always the top line.
- Dropping or flipping the sign. A −2.25 entered as 2.25 can be read as a plus. Enter the sign every time.
- Entering a CYL without its axis, or an axis without its CYL. They are a pair.
- Using a contact lens prescription. Different powers, different purpose.
- Adding the ADD yourself. For distance lenses, ignore it. For reading lenses, choose reading as the lens type and enter the prescription as written. The lab does the arithmetic.
Beyond those five, two habits help. Enter the values from a photo of the prescription rather than from memory, since most people misremember at least one number. And if any digit is unclear in the handwriting, call the practice rather than guess; they will read it to you over the phone.
Your prescription belongs to you
Under the FTC's Eyeglass Rule, an eye doctor in the US must give you a copy of your glasses prescription at the end of your exam, automatically and free of charge, whether or not you ask for it and whether or not you buy glasses from them. You can take it anywhere.
If you were not given one, or you have lost it, the practice is required to provide it on request. Similar rules apply in the UK, Canada and Australia.
Putting it to use
Once you can read the table, ordering lenses online is a matter of copying it across carefully. The lens types and index options available are explained when you add prescription lenses to a frame, and the measurements you need are on each product page.
Start with a frame
Titanium construction with a layered full-rim front, individually mounted nose pads and a curved bridge. The full rim hides and protects the lens edge, which matters more as a prescription gets stronger, and the pads let the fit be set once the lenses are in.
Browse the full optical eyewear collection, or read How to Buy Glasses Frames Online for how to match a frame to your prescription and check the fit before lenses go in. Prescription lenses can also be fitted to many sunglasses frames.
Frequently asked questions
What does OD and OS mean on a prescription? OD is the right eye and OS is the left. The right eye is always listed first. Some practices simply write R and L.
What does SPH mean? Sphere, the main strength of the lens in dioptres. A minus value corrects short sight, a plus value corrects long sight, and the further from zero the stronger the correction.
What does CYL and axis mean? Together they correct astigmatism. CYL is the amount of correction and axis is its orientation in degrees from 1 to 180. If one is present the other must be too.
What does ADD mean? Extra reading power for bifocal or progressive lenses. It is ignored when ordering single vision distance lenses.
Is a minus prescription bad? No. The sign only tells you the direction of correction. −1.50 is a mild short-sighted prescription; +1.50 is a mild long-sighted one. Neither is worse than the other.
Can I use my contact lens prescription to buy glasses? No. The powers are different because a contact lens sits on the eye rather than in front of it, and the contact lens prescription includes base curve and diameter values that glasses do not use.
My prescription has no PD. Is it still valid? Yes. PD is a measurement rather than part of the correction, and many optometrists do not write it down. You can measure it yourself or ask the practice for it. See How to find my PD.
How long is a glasses prescription valid? Typically one to two years in the US, depending on the state. Check the expiry date on your copy.
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