Visual acuity and 6/6 vision
Visual acuity is a measure of your central vision, the ability to distinguish details and shapes of objects.
Distant vision is tested with a chart with differently sized letters read from a distance of six metres away. This is called the Snellen’s Test Types.
Someone who can read the second line of letters up from the bottom is said to have 6/6 vision. This used to be called 20/20 vision when the test was done from 20 feet away before metric measures took over.
6/6 does not translate into perfect vision and does not indicate other important aspects of sight such as peripheral vision, the ability to identify colours or depth perception.
Having 6/12 vision means you can see at six metres what a person with normal vision can see at 12 metres away.
Each eye is tested separately - and an overall score is given for both eyes or binocular vision. So a person's Snellen test scores might be:
For driving, the Department of Work and Pensions says a person needs to be able to read the third line up from the bottom - or 6/9 vision. This can be with normal eyesight, or corrected with glasses or contact lenses.
Near vision is tested in a different way by looking at a test card with sentences of different font sizes.
Each paragraph is defined with 'points' to measure the print. A point is 1/72 of an inch. In most tests, N48 is the biggest type with N5 the smallest. The chart is read at a comfortable reading distance of about 35cm (14 inches) from the eyes.
Eye tests are important
To maintain good eye health and vision, it is important to have your eyes tested every two years, or more frequently if advised. You should seek advice immediately if you have any sudden loss of vision, eye pain, inflammation or irritation. Most eye diseases can be found during a routine eye test and treated when detected at an early stage. If you have health problems, such as diabetes, you may need to visit your optometrist more frequently to detect any complications.
What options are available to correct impaired vision?
There are many safe and affordable options for those in need of vision correction. They include:
- Glasses. Perhaps the most traditional form of vision correction, glasses improve vision by bending light. They are practical, affordable and safe.
- Contact lenses. Contact lenses are often chosen for cosmetic reasons or by people with an active lifestyle. There are many different types, colours and materials, so it is best to seek advice and shop around to find the contact lenses that work best for you.
- Corrective surgery. Vision correction surgery improves sight by changing the refractive, or light bending, properties of the eye. Refractive surgery restores a person's ability to see at a moderate visual acuity, meaning that there may still be a need in some cases for glasses or contact lenses.