Understanding Visual Acuity

Whether you use glasses or contact lenses – or are even considering Laser Eye Surgery – you will likely have heard the term ‘visual acuity’ a lot. This term essentially refers to the standard of your vision quality – that is, how well you see.

Your visual acuity is assessed by testing your central vision. For example, your ability to distinguish the details of objects at different distances. This is most commonly done using the Snellen Chart.

The Snellen Chart features rows of letters in variable sizes, getting progressively smaller on each line. To test your visual acuity, your optometrist will place the chart 20 feet (or 6 metres) away. You will then be asked to read the letters aloud until they become too small for you to clearly discern.

What does 20/20 mean?

Each line on the Snellen Chart has a label. One of the lower lines on the chart is the ’20/20′ line – a term many people are familiar with. Individuals who can distinguish the characters on this line are considered to have ‘normal’ visual acuity – they can see at 20 metres what a person with normal vision would be expected to see at that distance.

A person with 20/20 vision would be able to read the letters on the 20/40 line (three lines above) at a distance of 40 feet, and so on. If they cannot read the letters beyond this line, they have 20/40 vision.

But it is possible to have better than ‘normal vision’.

There are several lines beyond the 20/20 line on the Snellen Chart. Some people may be able to read the letters on the 20/16 line, meaning they can read at 20 feet what a person with normal visual acuity could only read at 16 feet away.

Visual acuity tests like the Snellen Chart also measure a person’s ‘best-corrected’ vision. While many people with myopia may struggle to read the letters on even the first or second line of the chart without glasses, they can often reach the 20/20 line with the help of their visual aids.

In most cases when visual acuity is mentioned, this refers to best-corrected visual acuity. For example, in the UK, drivers are required to have 20/40 visual acuity in order to legally drive. This means that you can drive if you have a best-corrected visual acuity of 20/40 or better – with glasses or contact lenses.

In most cases, Laser Eye Surgery aims to achieve the same visual acuity as your best-corrected level achieved with glasses and/or contact lenses. Many patients are even able to achieve better results!