OD, OS, SPH, CYL, AXIS, ADD, PD — your eyeglass prescription can look like a secret code. Our optometrists explain every term in plain language so you understand exactly what your eyes need.
First: What Is a Prescription?
An eyeglass prescription is a written order from a licensed optometrist that specifies the corrective lens power needed for each of your eyes. It's the result of a comprehensive eye examination and is unique to you. A prescription is valid for 1–2 years (as advised by your optometrist).
Understanding the Abbreviations
OD and OS
These are Latin abbreviations:
- OD = Oculus Dexter = Right Eye
- OS = Oculus Sinister = Left Eye
- OU = Oculus Uterque = Both Eyes (used when the same value applies to both)
Some modern prescriptions simply say "Right" and "Left." Either format is valid.
SPH (Sphere)
This is the lens power needed to correct your refractive error, measured in dioptres. It's always expressed in 0.25 increments.
- Minus (–) values correct myopia (short-sightedness). You struggle to see distant objects.
- Plus (+) values correct hyperopia (long-sightedness). You struggle to see close objects, or your eyes work harder to focus at all distances.
Example: SPH –2.50 means moderate short-sightedness. SPH +1.75 means mild long-sightedness.
CYL (Cylinder)
This value corrects astigmatism — an irregularity in the curvature of the cornea or lens. Not everyone has astigmatism, so CYL may be blank or written as "DS" (Dioptre Sphere, meaning no astigmatism).
Like SPH, CYL can be positive or negative, and is measured in 0.25 dioptre steps.
AXIS
If you have a CYL value, you'll also have an AXIS — a number between 1 and 180 degrees. It specifies the orientation of the astigmatism correction. The AXIS only matters if there is a CYL value; without CYL, AXIS is irrelevant.
ADD (Addition)
This value only appears on prescriptions for reading glasses or progressive (varifocal) lenses. It's the additional magnifying power needed for close vision, usually appearing for people over 40 who are experiencing presbyopia (age-related difficulty with near vision).
ADD values are always positive and typically range from +0.75 to +3.50.
PD (Pupillary Distance)
The distance in millimetres between the centres of your pupils. This measurement ensures the optical centres of your lenses align with your pupils for clear, comfortable vision. It may be written as a single number (binocular PD, e.g. 64mm) or two numbers (monocular PD, e.g. 32/32mm — right/left).
A Sample Prescription Decoded
Right Eye (OD): SPH –1.75 | CYL –0.50 | AXIS 170 | ADD +1.50
Left Eye (OS): SPH –2.00 | CYL –0.75 | AXIS 15 | ADD +1.50
PD: 63mm
This person has mild myopia (–1.75 and –2.00) with mild astigmatism (–0.50 and –0.75) in both eyes, and needs progressive lenses (ADD +1.50) for near vision. Their pupils are 63mm apart.
What If My Prescription Has Changed?
It's normal for prescriptions to change over time — especially in childhood, during pregnancy, or as we age. Annual eye exams ensure your prescription stays current. Wearing an outdated prescription causes unnecessary eye strain.
Can I Buy Glasses Without a Prescription?
Technically yes — for reading glasses (over-the-counter magnifiers) and purely fashion/non-prescription frames. But for any corrective lenses, you always need a valid prescription from a licensed optometrist. This protects your eyes and ensures accurate correction.
"Understanding your prescription puts you in control of your eye health. Never hesitate to ask your optometrist to explain it — it's your right to understand."
— Dr. Ankit Verma, Spectica Optometrist
Need a fresh prescription? Book a free eye test at Spectica and leave with a full prescription and expert frame advice.
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