01Purpose and principlesWhat the assessment is for and the core concepts behind it.
Visual acuity describes the ability to resolve detail at the point of fixation. A distance chart principally samples high-contrast central vision under controlled conditions; it does not measure the whole visual experience. A patient can read small letters yet struggle with glare, dim light, distorted lines, double vision or missing peripheral vision. Conversely, poor chart performance can result from an absent spectacle prescription, language or attention difficulties, as well as eye disease. Begin by asking what has changed and what the patient can no longer do, then establish a measurement whose conditions another clinician can reproduce.
Test the right and left eye separately, normally with the patient's distance glasses or usual contact lenses when their presence is safe and appropriate. Use the actual calibrated viewing distance of the chart. A chart designed for three metres cannot be treated as a six-metre chart merely because it looks similar. Check illumination, contrast and the patient's line of sight. Explain that guessing is acceptable, encourage an unhurried attempt and follow the chart's scoring convention. Prevent peeking around the occluder and avoid pressure that temporarily blurs the covered eye.
The Snellen fraction relates the testing distance to the distance at which the relevant line subtends its designated visual angle. In the common six-metre notation, 6/12 represents worse resolving ability than 6/6. A logMAR chart uses a logarithmic scale and usually offers more regular progression and letter spacing; a larger positive logMAR value indicates poorer acuity. Record the observed scale rather than improvising a conversion or treating unlike tests as directly interchangeable. Any change in chart, test distance, correction or method should accompany interpretation of a serial result.
A pinhole restricts peripheral light rays and can reduce optical blur. Position it accurately over the tested eye, retain the relevant distance correction and repeat the chart. A clear improvement makes a refractive contribution likely and may help estimate potential acuity before refraction. It is not a diagnosis of uncomplicated refractive error. Inadequate illumination, poor alignment, media opacity, irregular optics or retinal disease can limit the response. A person with significant eye disease may also have uncorrected refractive error, so improvement never cancels a concerning history or examination.
Near acuity addresses a different task. Ask about reading, phone use and the working distance needed for employment or daily activities. Use the patient's reading addition or appropriate habitual near correction and hold the selected near chart at its stated distance. Document the notation, distance and correction, including whether the patient read continuous text or identified isolated symbols. Reading difficulty can arise from presbyopia, central distortion, a field defect, impaired eye coordination or a neurological language problem; the near result must be interpreted alongside the wider assessment.
When measured vision is unexpectedly poor, first correct avoidable testing barriers rather than repeatedly asserting the same result. Check glasses, chart distance, occlusion, lighting, understanding and ocular comfort. Use a matching-symbol chart or suitable communication support when letter naming is unsuitable. If acuity remains reduced, establish its time course and compare with prior reliable records. Longstanding amblyopia may explain an asymmetry, but a remembered 'weak eye' should not automatically explain a new deterioration. Record uncertainty explicitly and arrange examination capable of identifying the cause.
Key points
- Measure each eye independently at the chart's specified distance, using habitual distance correction and documenting exactly what was worn.
- Keep the fellow eye fully covered without pressing on it; binocular testing alone can conceal marked unilateral impairment.
- Record the chart, viewing distance, right and left results, correction and any pinhole response rather than writing simply that vision is normal.
- Improvement through a pinhole supports an optical contribution, but does not exclude coexisting retinal, optic-nerve or other ocular disease.
- If letters cannot be read, document progressively simpler responses with their testing conditions, including counting fingers, hand movement and light perception.
- Test near vision with appropriate reading correction at the near chart's calibrated distance; distance acuity cannot substitute for a reading assessment.
- Interpret the result against symptom onset, previous measurements, fields, pupils and examination findings, including a possible emergency despite preserved central acuity.
02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.
Ask whether the reduction began suddenly, evolved over days or has been stable for years. Previous documented acuity is more informative than an assumption that an older patient normally sees poorly.
Missing glasses, an altered prescription or improved acuity through pinhole suggests an optical component. Continue assessing associated pain, distortion, field symptoms and the fundus when the presentation warrants it.
A patient describing a curtain, missing words on one side or collisions may have substantial field loss despite reading the chart well. Add fields and neurological assessment rather than relying on acuity.
Hearing loss, reduced literacy, language difference, cognitive impairment and motor limitations can alter performance. Choose an accessible test and record how the result was obtained without equating difficulty with non-cooperation.
03Method and interpretationA systematic approach to the test and its findings.
Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.
- 01
Monocular distance acuity with habitual correctionFirst step - Why
- Establish the presenting central visual function of each eye separately.
- Interpretation and limitations
- Use a calibrated chart and distance, record right and left scores and specify glasses, contact lenses or no correction. Habitual corrected acuity is not necessarily the best acuity achievable after formal refraction.
- 02
Pinhole reassessment of reduced distance vision - Why
- Identify whether reducing optical blur improves the measured result.
- Interpretation and limitations
- Centre the aperture and repeat under adequate lighting. An improvement supports an optical component; absent improvement is nonspecific and neither result independently establishes or excludes serious disease.
- 03
Structured assessment below the chart range - Why
- Describe severe visual impairment when standard optotypes cannot be resolved.
- Interpretation and limitations
- Use a validated closer-distance method where available, then record counting fingers at a stated distance, hand movements or light perception as appropriate. Do not assign an invented Snellen equivalent to these descriptions.
- 04
Near acuity with documented working distance - Why
- Assess detail needed for reading and other close tasks.
- Interpretation and limitations
- Record the near-chart notation and distance with the reading correction used. Compare like with like; moving the card closer changes angular size and is not an improvement under unchanged test conditions.
- 05
Focused ocular and neurological examination - Why
- Determine whether reduced acuity belongs to an optical, ocular or neurological process.
- Interpretation and limitations
- Select pupil testing, colour comparison, fields, eye movements, corneal assessment and fundal examination from the history. Investigations should explain the result rather than merely reproduce the numerical abnormality.
04Clinical next stepsHow the result changes management or prompts escalation.
01PreparationMake the measurement reproducibleFirst stepA patient presents with an eye complaint or needs a baseline visual assessment.+
- 1Identify the patient, explain monocular testing and check which distance correction they normally use, including whether contact lenses should be removed for the clinical problem.
- 2Position the patient at the chart's marked distance with suitable lighting and test each eye using an occluder that prevents peeking without applying pressure.
- 3Record the score using the chart's convention and document conditions that limit reliability, then repeat reduced results through pinhole when appropriate.
02Poor chart performanceClarify an unexpectedly low scoreThe patient cannot resolve the anticipated letters or gives inconsistent responses.+
- 1Recheck the prescription, alignment, chart distance and communication, offering matching symbols or another suitable method before interpreting the result as visual loss.
- 2If conventional acuity remains unmeasurable, describe the best demonstrable response and exact distance, assessing light perception carefully without repeated pressure or dazzling.
- 3EscalationCompare with earlier reliable results and examine for the cause, escalating promptly when the reduction is new, unexplained or accompanied by pain, field change or neurological symptoms.
03Functional follow-throughConnect distance and near findingsThe patient reports persistent difficulty despite a plausible distance acuity result.+
- 1Measure near performance with the relevant correction and ask about distortion, glare, intermittent blur, double vision and missing parts of the scene.
- 2Arrange refraction for an optical deficit and the appropriate eye or neurological assessment for unexplained findings, choosing urgency from the clinical presentation.
- 3Explain the result in practical terms, document a specific review plan and provide clear instructions for new pain, sudden deterioration or a curtain-like field symptom.
05Risks, monitoring and follow-upComplications, safety checks and further assessment.
- Use the same chart type, viewing distance and correction where possible at follow-up; otherwise state the differences before attributing a change to disease.
- Retest an unexpectedly altered result after addressing modifiable test conditions, but do not use repeated measurement to delay assessment of a convincing acute visual complaint.
- Document whether the patient can manage reading, medicines, mobility and work safely; functional difficulties may require support even when a numerical threshold appears satisfactory.
- Ensure the referral contains the result for each eye, baseline comparison, onset and relevant associated signs, with a named destination and an agreed urgency.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
Presenting and best corrected are different
Vision measured through the patient's current glasses is presenting corrected acuity. Best-corrected acuity usually implies optimisation by refraction; imprecise use of the term can conceal an outdated prescription or overstate the certainty of the assessment.
No chart result excludes every emergency
A small retinal tear, early field-threatening process or neurological visual deficit may spare fixation. The history determines whether additional examination is needed even when both eyes read the expected distance line.
Record severe loss without false precision
Counting fingers and hand-movement descriptions depend on distance, contrast and examiner technique. They convey clinically useful function but should not be converted into a precise chart fraction without a validated method.
Near tasks need their own correction
A presbyopic patient may have excellent distance vision and struggle at normal reading distance. Conversely, removing myopic distance glasses can aid near tasks; record this behaviour so the result is interpretable.
07Common pitfallsFrequent interpretation and management errors.
- 01
Recording a binocular score as evidence that each eye has satisfactory vision, thereby missing a unilateral deficit masked by the better eye.
- 02
Calling a pinhole improvement proof that urgent retinal or optic-nerve disease is absent despite a concerning new visual symptom.
- 03
Comparing a near chart used at two different distances without recording the change in conditions or the reading correction.
- 04
Labelling testing difficulty as functional visual loss before checking communication, cognition, suitable optotypes and possible ocular or neurological disease.