How to read your eye exam report: a patient's guide

You leave the eye doctor with a slip of paper or a patient-portal PDF covered in abbreviations: OD, OS, SPH, CYL, a number for axis, an add, maybe an IOP reading and a C/D ratio. Most people file it without ever decoding it. But your eye exam report is a compact medical record, and once you know the vocabulary, it tells you a surprising amount about your eyes and what your provider was watching.

This is a plain-English glossary. It will not turn you into your own optometrist, and it is not a substitute for asking your provider what a given number means for you. But it will let you read the page instead of just holding it.

TL;DR. OD = right eye, OS = left eye, OU = both. Your prescription has three main numbers per eye: SPH (overall lens power, minus for nearsighted, plus for farsighted), CYL and axis (the amount and orientation of astigmatism correction), and add (extra reading power in bifocals/progressives). Health measurements to notice: IOP (eye pressure, glaucoma-relevant) and C/D ratio (optic-nerve cupping). Your report almost certainly does not include a contrast sensitivity number — that axis of vision is rarely tested in a routine exam.

The two-letter codes: which eye

Everything on the report is organised by eye, using Latin abbreviations:

• OD (oculus dexter) — right eye.
• OS (oculus sinister) — left eye.
• OU (oculus uterque) — both eyes together.

Some modern reports have switched to plain RE / LE / BE (right, left, both). If you see a row per eye, that is what the two-letter prefix is telling you.

The prescription: SPH, CYL, axis, add

This is the part that becomes your glasses or contacts. Four columns do most of the work.

Sphere (SPH). The main lens power, in dioptres. A minus sign means you are myopic (nearsighted — distance blurry). A plus sign means you are hyperopic (farsighted). The further from zero, the stronger the correction: −1.00 is mild myopia, −6.00 is high myopia. "Plano," "PL," or "0.00" means no spherical power is needed.

Cylinder (CYL). The correction for astigmatism — a cornea or lens shaped more like a rugby ball than a football, so it focuses light unevenly. The number (also in dioptres, and usually written with a minus sign) is how much astigmatism correction you need. A blank or "DS" ("dioptres sphere") means essentially none. Astigmatism is extremely common and usually not a disease — just a shape.

Axis. A number from 1 to 180. It only appears when there is a cylinder value, and it describes the orientation of the astigmatism correction — which meridian the cylinder power sits on, in degrees. It is a direction, not a strength; 180 is horizontal, 90 is vertical. On its own it means nothing; it is the partner of CYL.

Add. Extra plus power for near work, added to the bottom of bifocal, trifocal, or progressive lenses. It shows up once you reach the age where the eye's own focusing muscle stiffens — presbyopia, usually starting in the mid-forties. A "+2.00 add" means two dioptres of reading boost. Both eyes almost always get the same add. If you want the longer story on why this happens, see our post on reading glasses versus genuine contrast loss.

Prism and base (direction) sometimes appear too — a correction for eye-alignment problems that cause double vision. Most prescriptions have none.

A worked example. A line reading OD −2.50 −0.75 x 90 add +2.00 decodes as: right eye, moderate nearsightedness (−2.50), a small amount of astigmatism (−0.75) oriented at the 90-degree meridian, plus a two-dioptre reading add. That is a fairly ordinary middle-aged prescription.

Visual acuity: the 20/20 line

Separate from the prescription, your report usually records visual acuity — how well you actually see, corrected and sometimes uncorrected. In the US this is the familiar Snellen fraction: 20/20, 20/40, and so on, where the bottom number is the distance at which a reference observer could read the same line. Research settings use logMAR instead (0.00 logMAR = 20/20), a scale built for measurement precision — we compare the two in logMAR vs Snellen.

Two cautions worth internalising. First, "cc" means with correction and "sc" means without; a great "20/20 cc" just means your glasses work, not that your eyes are problem-free. Second — and this is the whole reason our site exists — acuity is a high-contrast letter test. It can read a perfect 20/20 while contrast sensitivity, glare tolerance, or the visual field are quietly reduced. A clean acuity line is reassuring but not comprehensive.

The health numbers: IOP and C/D ratio

Beyond the refraction, your provider records measurements about the health of the eye. Two show up most often.

IOP — intraocular pressure. The fluid pressure inside the eye, in millimetres of mercury (mmHg), measured by tonometry (the puff of air, or a probe touching the numbed eye). A commonly cited "normal" range is roughly 10–21 mmHg, but the number is context-dependent: elevated pressure is a major risk factor for glaucoma, yet many people with glaucoma have normal-range pressure (normal-tension glaucoma), and many with slightly high pressure never develop it. One reading is a data point, not a verdict.

C/D ratio — cup-to-disc ratio. A description of the optic nerve head as seen on exam or imaging. The optic disc is where the nerve leaves the eye; the "cup" is the central hollow. The ratio (e.g. 0.3) is roughly the cup's diameter as a fraction of the disc. Larger cups and asymmetry between the two eyes can be a sign of glaucomatous nerve damage — which is why your provider notes it and watches it over time. Like IOP, a single value is interpreted in context, not in isolation.

You may also see notes on the lens (any early cataract), the macula (drusen or AMD changes), the retinal periphery, and tear film / ocular surface (dry eye). These are the observations, in shorthand, of everything your provider looked at.

What your report almost certainly leaves out

A routine exam and its report are built around refraction and ocular health. Several functional measures rarely appear:

• Contrast sensitivity — how faint a pattern you can detect. Not part of most standard exams, despite being reduced early in several conditions. This is the gap we built a free at-home test to help fill.
• Dark adaptation — how fast your night vision recovers, an early AMD signal.
• Detailed visual field, unless glaucoma or a neurological concern prompted formal perimetry.

If you have real-world symptoms — night-driving trouble, faces looking washed out, reading fatigue — that a "20/20, healthy eye" report does not explain, that mismatch is worth raising. It is often the functional axes above, not the ones on the slip, that carry the answer.

What this does and does not mean

Note. This glossary explains common notation; it does not interpret your report. Reference ranges (like 10–21 mmHg for IOP) are population statistics, not personal thresholds, and every number is read in the context of your history and the rest of the exam.

A "normal" report does not guarantee perfect vision, and an unfamiliar abbreviation is not a reason for alarm — it is a reason to ask. Your provider expects questions about their own notation.

A contrast sensitivity result — from us or anyone — is a screening signal, not a diagnosis, and belongs alongside the exam, never in place of it.

Frequently asked questions

What do OD and OS mean on my glasses prescription?

OD (oculus dexter) is your right eye; OS (oculus sinister) is your left eye; OU (oculus uterque) means both. Some newer reports use RE, LE, and BE instead.

What is a normal eye pressure (IOP)?

A frequently cited normal range is about 10–21 mmHg, but it is context-dependent. Glaucoma can occur at normal pressures (normal-tension glaucoma), and mildly elevated pressure does not always mean disease. Your provider interprets IOP alongside the optic-nerve appearance, visual field, and your risk factors — not as a stand-alone number.

What does the cup-to-disc (C/D) ratio tell me?

It describes the optic nerve head — the central "cup" as a fraction of the whole disc. A larger cup, an increase over time, or asymmetry between the two eyes can be a sign of glaucomatous damage, so it is tracked across visits. On its own, a single ratio is just a description; the trend and the context matter.

Why isn't contrast sensitivity on my report?

Because most routine exams do not test it. Standard exams centre on refraction (your prescription) and ocular health. Contrast sensitivity — how faint a pattern you can see — is a separate functional axis that is reduced early in several conditions but is rarely measured unless specifically requested. You can screen it yourself with a free at-home test and bring the result to your provider.

My report says 20/20 but my vision still feels off. Is that possible?

Yes. 20/20 is a high-contrast letter score. It can be normal while contrast sensitivity, glare recovery, focusing stamina, or the visual field are reduced. A mismatch between a clean acuity and real-world symptoms is worth raising with your provider — see why a single acuity number is misleading.

Take the test

Your eye exam report is thorough on refraction and eye health. It is usually silent on contrast sensitivity — the axis of vision that governs faces, fog, dusk, and low-contrast reading.

Take the test now. It is free, runs in your browser in about three minutes, and produces a number you can save and bring to your next appointment. See the methodology page for how it is calibrated, and our guide to bringing a contrast result to your optometrist for how to use it in the room.

References

• Bailey, I. L., & Lovie, J. E. (1976). New design principles for visual acuity letter charts. American Journal of Optometry and Physiological Optics, 53(11), 740–745. The design basis for the logMAR/ETDRS charts that underpin research-grade acuity scoring referenced in this guide.
• Kaiser, P. K. (2009). Prospective evaluation of visual acuity assessment: a comparison of Snellen versus ETDRS charts in clinical practice (An AOS Thesis). Transactions of the American Ophthalmological Society, 107, 311–324. Documents how the Snellen acuity on a clinic report differs from standardised research acuity.
• Pelli, D. G., Robson, J. G., & Wilkins, A. J. (1988). The design of a new letter chart for measuring contrast sensitivity. Clinical Vision Sciences, 2, 187–199. The Pelli-Robson chart — the standard clinical contrast-sensitivity instrument, a measure absent from most exam reports.
• Mäntyjärvi, M., & Laitinen, T. (2001). Normal values for the Pelli-Robson contrast sensitivity test. Journal of Cataract and Refractive Surgery, 27(2), 261–266. Age-stratified normative contrast-sensitivity values — the reference context a report typically lacks.