Understanding visual acuity
‘Visual acuity’ is the standard measure of a person’s ability to see clearly.
When your visual acuity is measured, your optometrist is testing your central vision – your ability to distinguish the details and shapes of objects from a certain distance away.
What does 20/20 mean?
It is possible to see better than the norm – better than 20/20
In the most common visual acuity test, an optometrist places an eye chart at a standard distance (twenty feet or six metres). The patient is then asked to read letters on lines across the eye chart. Towards the bottom of the chart is a line known as the ‘20/20 line’ – this is the smallest line that a person with ‘normal’ visual acuity can read from twenty feet away. Therefore, having ‘20/20 vision’ simply means that you have normal visual acuity.
Three lines above the normal (20/20) line is the 20/40 line. The letters on the 20/40 line are twice as large as those on the 20/20 line, so a person with normal visual acuity could read from twice as far away – forty feet.
If the smallest letters that a person can read from twenty feet away are those on the 20/40 line, that person would be said to have 20/40 vision. (This means that the person needs to approach a distance of twenty feet to read letters that a person with normal acuity could read at forty feet.)
It is also possible to have better than 20/20 vision.
For example, some patients are able to read the 20/16 line (the line below the 20/20 line) during their visual acuity test. They would be said to have a visual acuity of 20/16, which means that from twenty feet away, they can read a line that a person with normal visual acuity would need to approach to a distance of sixteen feet to read.
The biggest letter, at the top of the eye chart, is the 20/200 line. Many people believe that they have “bad vision” because they “can’t even read the E at the top of the chart without glasses.”
However, importantly, visual acuity tests measure not only a person’s uncorrected vision (their vision without glasses or contact lenses) but also their best-corrected vision – the best vision that they are able to achieve with glasses or contact lenses.
Many people with moderate myopia (short-sightedness) cannot read the ‘E’ without glasses but have no problem reading the 20/20 line or even the 20/16 line with glasses.
In most situations where visual acuity ratios are mentioned, they refer to best-corrected visual acuity. For example, the legal driving standard in the UK is 20/40, which means that a person’s visual acuity is considered good enough to drive if they can read the 20/40 line from twenty feet away, with glasses on if necessary.
In laser eye surgery, the surgeon’s goal is to get your visual acuity without glasses to the same level as your best-corrected visual acuity (i.e. your vision with glasses or contact lenses) before surgery.
Occasionally, patients achieve even better vision than this.