Reading an eyeglass prescription is easier when you separate eye labels from lens power. The goal is not to diagnose your vision yourself. The goal is to enter the numbers correctly when you choose frames and lenses online.
This guide explains OD, OS, SPH, CYL, cylinder power, Axis, ADD, PD, diopters, farsightedness, nearsightedness, and the checks that matter before ordering glasses online.
What OD and OS mean
OD refers to the right eye. OS refers to the left eye. Some prescriptions may also use RE and LE. OU means both eyes. Always enter each value under the matching eye when ordering.
Common prescription terms
| Term | What it means | What to check |
|---|---|---|
| SPH | Sphere power for nearsightedness or farsightedness | Copy the plus or minus sign exactly |
| CYL | Cylinder power for astigmatism | May be blank, zero, plus, or minus |
| Axis | Direction of astigmatism correction | A number from 1 to 180 when CYL is present |
| ADD | Extra near power for reading, bifocal, or progressive lenses | Usually a positive number |
| PD | Pupillary distance | Single PD or dual PD format |
| Prism | Special correction for eye alignment | Follow the written prism amount and base direction |
What is a diopter?
A diopter is the unit used for lens power. On a prescription, SPH, CYL, and ADD values are written in diopters. A small number such as -0.50 is a mild correction. A larger number such as -6.00 is a stronger correction and may need a more careful frame and lens-index choice.
Diopter meaning is simple in practice: it describes how much focusing power the lens needs. Do not change a diopter value yourself. Enter the number exactly as written on the current eyeglass prescription.
Searches like glasses -4.00, corrective glasses, prescribe glasses, vision eyeglasses, or what does sphere mean for glasses usually point to the same prescription-reading step. SPH, or sphere, is the main lens power. A minus value is commonly associated with nearsighted correction, while a plus value is commonly associated with farsighted correction.
What does cylinder mean for glasses?
Cylinder is the full word behind CYL on many eyeglass prescriptions. It describes the astigmatism correction, while Axis describes the direction of that correction. If your prescription has a cylinder value, copy the matching Axis value too; the two fields work together.
Farsighted vs nearsighted prescriptions
Negative SPH values usually indicate nearsightedness, also called myopia. This means distance vision needs correction. Positive SPH values usually indicate farsightedness, also called hyperopia. This means near vision can be harder, although the exact experience varies by person and age.
If you see searches like farsighted meaning, far sighted means, short sighted spectacles, or define hyperopia, they usually relate to the plus or minus side of SPH. If you see high myopia or thick lenses, that usually relates to stronger minus values and a need to compare smaller frame shapes and high-index lenses.
How to choose the right lens type
Single-vision lenses are used when the glasses correct one viewing distance. Progressive lenses are used when you need distance, intermediate, and near support in one lens. Reading glasses focus on near tasks. Bifocal lenses use two zones, while progressive lenses use a gradual change without a visible line.
If you are unsure which lens type your prescription requires, ask your eye-care professional before ordering. Lens type should follow how you use your glasses, not just the keywords on a product page.
Lens index and stronger prescriptions
Lens index describes how efficiently the lens material bends light. Higher-index lenses can be thinner than standard lenses for the same prescription. This matters most for stronger prescriptions, especially high myopia, where edge thickness can be more visible.
If you are comparing lens numbers such as 1.50, 1.56, 1.59, 1.60, 1.61, 1.67, or 1.74, those numbers refer to the refractive index of the lens material. They are not frame sizes. A higher number usually means the material can bend light more efficiently, which may allow a thinner lens for the same prescription.
| Search intent | What to compare | Why it matters |
|---|---|---|
| 1.56 index lenses | Basic lens thickness and price | Often suitable for lower prescriptions |
| 1.59 index lenses | Commonly associated with polycarbonate lens material | Often compared for lightweight and impact-resistant everyday use |
| 1.60 index lenses | Mid-index lens option near 1.61 | Often compared when shoppers want a thinner profile without jumping to very high index |
| 1.61 high index lenses | Thinner profile than basic lenses | Useful for moderate prescriptions |
| 1.67 high index lenses | Thinner and lighter appearance | Common for stronger prescriptions |
| 1.74 high index lenses | Very thin lens option | Often considered for high prescriptions |
| Thick lenses | Frame size, rim type, and material | Small full-rim frames usually hide thickness better |
For searches like 1.50 vs 1.60 index lenses, 1.57 vs 1.61 lenses, or 1.59 vs 1.67 index lenses, the practical difference is usually less important than the prescription strength, frame size, rim type, and what lens materials are available during checkout. Do not choose by index number alone; match the lens index to your prescription and frame.
For stronger prescriptions, read the high myopia frame guide before choosing oversized, rimless, or aviator-style frames.
Aspheric vs spherical lenses
Aspheric lenses and spherical lenses describe lens design, not the prescription values themselves. A spherical lens uses a simpler curved surface. An aspheric lens uses a flatter, more complex surface that may reduce thickness or magnification appearance for some prescriptions. Availability depends on the lens material, index, prescription, and the selected frame.
If you are comparing aspherical lenses, mid-index lenses, or high-index lenses, start with the prescription strength and frame size first. The best lens design is the one that matches your optical needs, fit, and checkout options rather than the most technical term in isolation.
Broad searches such as optical lens, optical lenses, glasses lens, lense glasses, thickness glasses, thin glasses, or lenticular lenses usually point to the same lens-selection concern: how prescription strength, lens design, frame size, and material affect the final pair. Very specialized lens designs are not always part of a standard online glasses order, so check the available lens package and ask a professional when the prescription is complex.
How long is an eye prescription good for?
Prescription validity depends on local rules and your eye-care provider. Many eyeglass prescriptions are written for one to two years, but the expiration date on your prescription is the value that matters. If your prescription is expired, blurry, uncomfortable, or no longer feels accurate, get a new eye exam before ordering.
Common mistakes to avoid
- Do not switch OD and OS.
- Do not drop a plus or minus sign.
- Do not enter Axis without checking the matching CYL value.
- Do not guess PD if your measurement is inconsistent.
- Do not use a contact lens prescription as an eyeglass prescription.
- Do not assume the thinnest lens is always necessary for every prescription.
Where to go next
After checking your prescription, browse all glasses, compare lens options, read the PD guide, or review the frame guide for thick lenses before placing an order.
FAQ
What does SPH mean on a glasses prescription?
SPH stands for sphere and shows the main lens power for nearsightedness or farsightedness.
What does CYL mean?
CYL stands for cylinder and is used when the prescription includes astigmatism correction.
What does Axis mean?
Axis shows the direction of astigmatism correction and is usually listed as a number from 1 to 180.
What does ADD mean?
ADD is extra near power, often used for reading, bifocal, or progressive lenses.
What is a diopter in glasses?
A diopter is the unit used to measure lens power. SPH, CYL, and ADD values are all written in diopters.
Can I order glasses with an expired prescription?
You should use a current eyeglass prescription. If the prescription is expired or your vision has changed, get an updated eye exam before ordering.