Visual Acuity
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Direct answer
6/6 means the patient reads at 6 metres a line a normal eye manages at 6 metres — the Snellen fraction, testing distance over the distance at which the letter subtends 5 minutes of arc with a limb width of 1 minute. Test each eye separately, unaided then with present correction, record the smallest line read (more than half its letters), and use the pinhole to separate refractive blur from organic disease: improvement through a 1–1.5 mm pinhole points to refraction; failure to improve points to amblyopia or pathology. Below the top line, vision is walked down the ladder of counting fingers, hand movements, perception of light and its projection.
What you must remember
- Snellen fraction V = d/D; 6/6 equals 20/20 in feet and 1.0 in decimal; a 6/60 letter subtends 5 minutes of arc at 60 metres.
- The 6-metre test distance approximates optical infinity: rays are effectively parallel and accommodation demand falls to about 0.17 D.
- Record the line read plus letters missed (6/9−2), unaided and aided, each eye separately with the fellow eye fully occluded.
- Pinhole: acuity improves in uncorrected refractive error; fails to improve in amblyopia, macular disease and optic neuropathy — dense cataract may even worsen it.
- logMAR charts (ETDRS/Bailey–Lovie): five letters per line in uniform 0.1 log steps — 6/6 = 0.00, 6/60 = 1.00; the standard for research and low-vision work.
- If the top line is unreadable at 6 m, walk forward metre by metre (5/60, 4/60…), then counting fingers at a stated distance, hand movements, then perception of light with projection in four quadrants.
- Near vision uses N-notation (N-6 approximates newsprint at 35–40 cm) or Jaeger types, always with the working distance stated.
- WHO categories (presenting acuity, better eye): moderate below 6/18 to 6/60, severe below 6/60 to 3/60, blindness below 3/60 or field under 10 degrees — the classification behind India's National Programme for Control of Blindness reporting.
Two patients, one pinhole, one decision
A 52-year-old labourer reports blurred vision for a year. Uncorrected acuity is right 6/60, left 6/36. Through the pinhole he jumps to 6/9 in both eyes — refractive error, most likely uncorrected astigmatism or hypermetropia; the next step is refraction, not referral. Beside him, a 60-year-old diabetic reads 6/36 unaided and still 6/36 through the pinhole. The pinhole has just said the media are not the whole story: maculopathy, cataract or optic nerve — the pathway is slit-lamp and fundus examination.
The recording discipline carries the clinical logic. For the labourer: "VA RE 6/60, improves to 6/9 with pinhole; LE 6/36, 6/9 with pinhole." For the diabetic: "VA RE 6/36, no improvement with pinhole." A reader — consultant, court or examiner — reconstructs both diagnoses from those two lines. Contrast the habit of writing "vision poor", or testing both eyes open so the better eye does all the work — a dense amblyope is found one occlusion at a time.
Where students slip
The traps are numerical and terminological. Converting between systems badly: 6/18 is logMAR 0.5, not 0.18 — it is the log of the reciprocal of the decimal fraction. Defining blindness as "cannot see" — in WHO and Indian programme terms it is presenting acuity below 3/60 in the better eye or a field under 10 degrees, and the shift from best-corrected to presenting acuity matters because uncorrected refractive error now counts. Recording "PL+" without projection: an eye with intact projection has a healthy retina behind the opacity and may retrieve vision after cataract surgery, while absent projection forecasts a poor outcome — the most prognostic piece of the entire low-vision exam. And the top-line rule: someone who cannot read 6/60 at 6 metres is walked closer and recorded 5/60 or 4/60, never "hand movements" straight from the chair.
Frequently asked questions
Why is visual acuity tested at 6 metres?
Because 6 metres approximates optical infinity: entering light is effectively parallel, so accommodation is minimally engaged (about 0.17 D) and distance vision is tested without near effort.
What does improvement with a pinhole indicate?
Refractive error or early media opacity — a narrow pencil of rays bypasses blurred peripheral focus; no improvement suggests amblyopia or retinal and optic nerve disease.
What is the logMAR value of 6/18?
0.5, since logMAR is the logarithm of the inverse of the decimal acuity (6/18 = 0.33 decimal; log 3 ≈ 0.5).
How is vision recorded below the 6/60 line?
By walking the patient forward (5/60, 4/60, 3/60…), then counting fingers at a stated distance, hand movements, and perception of light with its projection in four quadrants.
What are the WHO categories of visual impairment?
Based on presenting acuity in the better eye: moderate below 6/18 to 6/60, severe below 6/60 to 3/60, blindness below 3/60 or field under 10 degrees.
How is acuity tested in pre-literate children?
With matching or picture optotypes (Lea symbols, tumbling E, HOTV) and, in infants, behavioural methods — optokinetic nystagmus and preferential looking with Teller acuity cards.