Visual acuity is the sharpness of your central vision — your ability to make out fine detail. It is the vision measurement most people know from eye charts.
How it is tested
You read letters or symbols of decreasing size on a chart at a set distance, covering one eye at a time and then using both eyes. Common charts include the Snellen chart and the more standardised logMAR charts. The smallest line you can read reliably gives your result.
Distance, intermediate and near
- Distance — lookout, recognising objects and lights.
- Intermediate — reading instruments and displays.
- Near — charts, documents and gauges close up.
What can affect your result
- Uncorrected refractive error (short- or long-sightedness, astigmatism)
- Age-related changes in focusing (presbyopia), usually from the mid-40s
- Tiredness, dry eyes or poor lighting
- Out-of-date glasses or scratched lenses
Getting an accurate measurement
Have an eye test with an optometrist or ophthalmologist before your medical if you have not had one recently, bring your current glasses or lenses, and arrive rested.
Sources
- World Health Organization — Blindness and vision impairment — fact sheet (opens in a new tab)
- International Maritime Organization — STCW Convention and Code — Regulation I/9, Section A-I/9 (opens in a new tab)
Links go to the original publishers. SeafarerFit is not affiliated with or endorsed by these organizations.
SeafarerFit provides educational information, health calculators, screening tools and medical-examination preparation resources. It does not provide diagnosis, treatment, medical certification or official determination of fitness for sea service.