The visual acuity (Va) is a measure of how fine your vision is, and is measured in adults by your ability to read various sized letters in a chart presented to you at a specified distance.

The smaller the letters you are able to read, the better your visual acuity. So, the smaller the bottom number on your Snellen Acuity measurement, the better your visual acuity is.

Most charts are presented at a distance equivalent to 20 feet or 6 metres. That is why the Va is often written, for example as 20/20 or 6/6. The charts most commonly used are Snellen Charts, and so this measurement is often referred to as 'Snellen Acuity'.

A Va of 6/6 is equivalent to what many may call 'perfect vision' or '20/20 vision'.

A Va of 6/12 means that at 6m, the smallest letter you can see can be seen by somebody with '20/20' vision at 12 metres. Hence, 6/12 acuity is not as good as 6/6.

A Va of 6/4 means that at 6m, the smallest letter you can see can be seen by somebody with '20/20' vision at 4 metres. Hence, 6/4 acuity is better than 6/6. Some people therefore can see better than 20/20.