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Visual acuity: Snellen, logMAR and pinhole

  • Examination

Visual acuity is the vital sign of the eye: each eye is measured separately, with glasses, and repeated through a pinhole if it is low.

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What it is

Visual acuity measures the smallest detail each eye can resolve at a set distance. It is the first measurement in any eye complaint and the basis for referral, follow-up and medico-legal records. The Snellen chart gives a fraction (6/6 or 20/20 is normal); logMAR charts (ETDRS) are more precise and used in research and low vision. The pinhole separates refractive blur from disease.

Normal values and scales

  • Normal adult acuity: 6/6 (20/20) or better; the Snellen numerator is the test distance and the denominator the distance at which a normal eye reads that line.
  • logMAR 0.0 equals 6/6; each line is 0.1 logMAR, and higher numbers mean worse vision.
  • Below the chart: counting fingers, hand movements, light perception, no light perception.
  • WHO grades of distance visual impairment: mild worse than 6/12, moderate worse than 6/18, severe worse than 6/60, blindness worse than 3/60.
  • A two-line difference between the eyes of a child suggests amblyopia.

How it is done

  1. Seat the patient at the chart's distance (usually 6 metres, or 3 metres with a mirror or a reduced chart), wearing their distance glasses.
  2. Cover one eye fully with an occluder or the palm, without pressing, and test the other eye; then swap.
  3. Record the smallest line read with no more than one or two mistakes, as a fraction (for example 6/9 or 20/30).
  4. If the top letter cannot be read, walk the patient closer and record the distance, then try counting fingers, hand movements and light perception.
  5. If acuity is below 6/6, repeat looking through a pinhole occluder.
  6. Test near vision with a reading card at the patient's usual reading distance.
  7. In young children, use picture or matching charts (Lea, HOTV), or fixing and following in infants.

How to interpret it

  • Improvement with the pinhole means a refractive error; no improvement means disease of the media, retina or optic nerve.
  • Acuity tests central (foveal) vision only: a patient with advanced glaucoma can read 6/6.
  • Always record which eye, with or without glasses, and with pinhole: the record is used to compare over time and in medico-legal reports.

Next step / Initial management

  • Acuity worse than 6/12 that improves fully with the pinhole: refraction by optometry.
  • Acuity worse than 6/12 that does not improve with the pinhole: ophthalmology, with priority set by the history (same day if sudden).
  • Child with reduced acuity in one eye or a two-line difference: paediatric ophthalmology for amblyopia.
  • Sudden loss of acuity, or loss with pain: same-day ophthalmology.

Test yourself

Think of the answer, then tap the question to see it.

What does a Snellen acuity of 6/18 mean?

The patient reads at 6 metres the line that a normal eye reads at 18 metres.

Acuity improves from 6/24 to 6/9 with a pinhole. What is the cause?

A refractive error, which glasses would correct.

How does WHO define blindness by visual acuity?

Presenting acuity worse than 3/60 in the better eye.

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Educational content; not a clinical guideline and no substitute for assessment by a health professional. For sudden visual loss, severe eye pain, a chemical burn or an eye injury, go to an emergency department at once.

Sources: Evaluation of Visual Acuity (StatPearls Publishing (NCBI Bookshelf), 2024); World report on vision (World Health Organization, 2019).

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