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Your eye prescription is a small slip of paper that holds a significant amount of information — and if you've ever looked at it and wondered what the numbers and abbreviations actually mean, you're in good company. When ordering glasses online, understanding your prescription is the difference between getting exactly the lenses you need and making an error that affects how well your glasses work. This guide walks through every element of a standard prescription, what each one means, and what to enter where when placing an order.
Prescription Terms at a Glance
| Term | What It Stands For | What It Means |
|---|---|---|
| OD | Oculus Dexter | Right eye |
| OS | Oculus Sinister | Left eye |
| OU | Oculus Uterque | Both eyes |
| SPH | Sphere | Main lens power — how much correction is needed for distance or near vision |
| CYL | Cylinder | Astigmatism correction — how much, if any |
| AXIS | Axis | Orientation of the astigmatism correction — a number between 1 and 180 |
| ADD | Addition | Extra near power for reading — used in bifocal and progressive prescriptions |
| PD | Pupillary Distance | Distance between the centres of both pupils — critical for lens centration |
| PRISM | Prism | Corrects eye alignment issues — not present in most prescriptions |
| BASE | Base Direction | Direction of prism correction — only relevant when PRISM is prescribed |
Key Things to Know Before Entering Your Prescription Online
- OD is always the right eye and OS is always the left — this order is standard across all prescriptions globally
- SPH values with a minus sign (−) indicate short-sightedness (myopia); values with a plus sign (+) indicate long-sightedness (hyperopia)
- If the CYL column is blank or shows "0.00", you do not have astigmatism — leave it blank or enter 0
- AXIS only applies when CYL has a value — without cylinder power, axis is meaningless and should not be entered
- ADD is only required for progressive and bifocal lenses — it is not part of a standard single vision prescription
- PD is one of the most critical values for correct lens performance — if it is missing from your prescription, measure it or request it from your optometrist
- A prescription that is more than two years old may no longer reflect your current vision accurately — a fresh eye test before ordering is always recommended
The Complete Guide: Reading Your Eye Prescription
What a Prescription Actually Is
An eye prescription is a clinical document written by an optometrist or ophthalmologist after a comprehensive eye examination. It specifies the precise optical correction your eyes need to produce clear, comfortable vision. Each value on the prescription corresponds to a specific aspect of how your lenses need to be made — the power, the shape, the orientation, and the positioning.
No two prescriptions are identical, and the values on yours are specific to your eyes alone. This is why using someone else's prescription — or estimating values based on how your current glasses feel — produces lenses that don't work correctly. The prescription is the foundation of the entire pair, and entering it accurately is the single most important step in an online order.
OD and OS: Right Eye and Left Eye
Most prescriptions are presented in a table format with two rows — one for each eye. OD, from the Latin oculus dexter, refers to the right eye and always appears first. OS, from oculus sinister, refers to the left eye and appears second. Some prescriptions use "RE" for right eye and "LE" for left eye — these mean exactly the same thing.
This order — right eye first, left eye second — is consistent across every prescription you'll encounter. When entering values online, always confirm which field corresponds to which eye before filling in the numbers. Swapping the values between eyes is one of the most common ordering errors and produces lenses that are the optical opposite of what each eye needs.
SPH (Sphere): The Main Lens Power
The sphere value is the primary power of the lens — it tells you how much optical correction the lens needs to provide and in which direction. This value is always written with either a minus (−) or plus (+) sign, and that sign is as important as the number itself.
A minus sphere value — such as −2.00 or −4.50 — indicates short-sightedness, or myopia. Myopic eyes see clearly up close but blur at distance. The minus lens diverges light to compensate, moving the focal point back to the retina. A higher minus number means stronger short-sightedness.
A plus sphere value — such as +1.75 or +3.00 — indicates long-sightedness, or hyperopia. Hyperopic eyes have difficulty focusing at close range. The plus lens converges light to bring the focal point forward. A higher plus number means stronger long-sightedness.
When no sphere correction is needed for an eye, the prescription may show "Plano," "PL," or "0.00." This means no spherical correction is required for that eye — not that the field should be left empty.
SPH values are written in increments of 0.25 dioptre. Common values include −1.00, −1.25, −1.50, and so on. When entering online, include the correct sign — entering −2.50 as 2.50 without the minus is an error that will produce completely the wrong lens.
CYL (Cylinder) and AXIS: Astigmatism Correction
Cylinder and axis work together to correct astigmatism — an irregularity in the curvature of the cornea or lens that causes blurred or distorted vision at all distances. Not everyone has astigmatism. If the CYL column on your prescription is blank or shows a dash, you don't have astigmatism, and neither cylinder nor axis needs to be entered for that eye.
The cylinder value represents how much astigmatism correction is needed. Like sphere values, it can be either minus or plus, depending on the notation system your optometrist uses. Minus cylinder notation is more common, but plus cylinder notation is also used — if you've had prescriptions from different optometrists and the numbers look different, this may be why. The optical result is the same; only the notation differs. Most online ordering systems use minus cylinder notation by default.
The axis value is a number between 1 and 180 that indicates the orientation at which the cylinder correction must be applied. It represents degrees on a circle — 90 is vertical, 180 is horizontal. Axis only means something when there is a cylinder value to go with it. Without cylinder power, axis has no function and should not be entered. Entering axis without a corresponding cylinder value can confuse automated prescription processing systems.
A common entry on an astigmatism prescription looks something like: SPH −1.75, CYL −0.75, AXIS 160. This means the main power is −1.75 for distance, with −0.75 cylinder correction applied at 160 degrees to address the astigmatism component.
ADD (Addition): For Progressive and Bifocal Prescriptions
The ADD value appears on prescriptions for wearers who need correction at more than one distance — most commonly from the mid-40s onward when presbyopia develops. It represents the additional plus power added to the distance prescription to produce clear near vision. It is only relevant for progressive, bifocal, and reading glasses — not for single vision distance lenses.
ADD values are always positive and typically range from +0.75 to +3.50 in increments of 0.25. A lower ADD — around +0.75 to +1.25 — suggests early presbyopia. A higher ADD — +2.00 and above — indicates more advanced presbyopia where near vision without correction is significantly impaired.
In most prescriptions, the ADD value is the same for both eyes — presbyopia typically affects both eyes symmetrically. When the ADD is listed once at the bottom of the prescription table rather than in individual eye rows, it applies to both eyes equally.
If you are ordering single vision distance glasses, ignore the ADD column — it does not apply. If you are ordering progressive lenses, the ADD is required and must be entered accurately. ELUNO's progressive lenses — Wide Corridor, Wide Pro Corridor, and Wide Max Corridor — are designed to work with the full range of ADD values, with wider corridor designs performing particularly well for moderate to higher ADD prescriptions.
PD (Pupillary Distance): The Value Most People Overlook
Pupillary distance is the measurement in millimetres between the centres of the two pupils. It is used to centre each lens precisely in the frame so that the optical centre of the lens aligns with the wearer's pupil. Correct PD is fundamental to how well prescription lenses work — a lens that is centred even two or three millimetres off from the correct pupillary position introduces prismatic effect that causes eye strain and, in sensitive wearers, headaches.
PD is often the most confusing element of online ordering because it is sometimes missing from prescriptions. Not all optometrists include it automatically — some consider it a dispensing measurement rather than a prescription component. If your prescription doesn't show a PD value, request it specifically from your optometrist before ordering. It can also be measured at home with a ruler and a mirror, or by a friend with a standard ruler — instructions for this are widely available and the process is straightforward for most people.
PD is expressed in one of two ways. A single number — such as 64 — is the full binocular PD, the total distance between both pupils measured as one value. This is the most common format. Two numbers — such as 32/31 — represent monocular PDs: the distance from each pupil to the centre of the nose bridge, right eye first. Monocular PDs are more precise and are preferred for progressive lenses and stronger prescriptions where even small centration errors have a noticeable effect.
Average adult PD falls roughly between 58 and 72 millimetres, with women averaging slightly lower than men. If the number you have falls significantly outside this range, it is worth double-checking the measurement before ordering.
PRISM and BASE: Less Common but Important
Prism correction is prescribed for wearers with binocular vision problems — conditions where the two eyes do not align correctly, causing double vision or significant eye strain. Prism power is measured in prism dioptres and redirects light to compensate for the misalignment. The BASE value indicates the direction of the prism — base in, base out, base up, or base down.
Most people do not have a prism prescription. If your prescription shows no PRISM column or the prism fields are blank, you do not need to concern yourself with this. If prism is prescribed, it is essential to enter it correctly — entering prism incorrectly or omitting it from an order produces lenses that not only don't correct the binocular issue but can actively worsen it.
Prism prescriptions require careful handling and are worth discussing directly with an optical professional before ordering online to confirm the entry is correct.
How Prescription Notation Can Vary
One source of confusion when reading prescriptions is that notation can vary slightly between optometrists and between countries. The core values — SPH, CYL, AXIS, ADD, PD — are universal, but the way they are laid out on the page, the column headers used, and whether values are shown for both eyes or combined can differ.
Some prescriptions show the right eye values at the top and left eye below. Others show them in columns side by side. Some use R and L instead of OD and OS. Some combine the sphere and any existing reading correction differently. If a prescription format is unclear, the safest approach is to contact the optometrist who issued it and confirm which value is which before entering anything online.
A prescription that shows only one set of values — no separate OD/OS rows — may be a monocular prescription for one eye only, or it may be a reading-only prescription for both eyes without a distance component. Context matters here, and if in doubt, confirming with the issuing optometrist takes only a few minutes and prevents an ordering error.
When a Prescription Is Too Old to Use
Eye prescriptions expire. In India, a standard prescription is typically valid for one to two years, depending on the issuing optometrist's recommendation and any conditions that may cause faster change — such as diabetes or a history of rapid prescription shifts. Using an outdated prescription for an online order means ordering lenses based on what your eyes needed previously, not what they need now.
If your prescription is more than two years old, an updated eye examination before ordering is the right approach. Vision changes gradually and often imperceptibly — many people don't notice their prescription has shifted until they put on correctly updated lenses and the improvement is immediately clear. An up-to-date prescription means lenses that work for your eyes as they are today, not as they were two years ago.
Ordering ELUNO Lenses with Your Prescription
When ordering from ELUNO, having your prescription values clearly in front of you before beginning the process makes the experience straightforward. Enter the SPH, CYL, and AXIS values exactly as they appear on your prescription — including the correct sign for each. Enter your PD value — monocular PDs if available, binocular if not. For progressive lenses, include the ADD value.
Every ELUNO lens is made with the full set of Essential Coatings included as standard — anti-reflective coating, UV and blue light protection, scratch resistance, water repellent, smudge resistance, and dust resistance — so there is no need to select coatings separately. The index decision — 1.56, 1.60, 1.67, or 1.74 — is the main variable to consider based on your prescription strength and frame choice. ELUNO's lens guide covers each index option and helps match the right one to your prescription.
If you are unsure about any aspect of your prescription or what to enter, the team at ELUNO stores is available to assist — either in person or through the care contact. Getting the prescription entry right from the outset means lenses that work exactly as they should from the first day of wear.
Whether you are choosing from ELUNO's eyeglasses collection or exploring options for a specific lifestyle or prescription need, a clear prescription in hand is the starting point for a pair of glasses that genuinely delivers on everything it should.
Final Thought
Reading your eye prescription is not complicated once you know what each element means — and understanding it puts you in full control of any online eyewear order you make. The key values are SPH, CYL, AXIS, ADD if relevant, and PD. Enter them accurately, include the correct signs, and confirm the right eye and left eye values are in the right fields. Done correctly, online ordering is a reliable and convenient way to get exactly the lenses your eyes need in exactly the frame you want.
At ELUNO, the prescription you provide is the foundation of every pair. The lenses are made precisely to those values, with the full range of Essential Coatings applied as standard. Your prescription is the input. A pair of glasses that works perfectly from day one is the output — and that's exactly what it should be.
