- A diopter is a unit of measurement that determines your eye power. A minus sign means near-sightedness, a plus sign means far-sightedness.
- 20/20 (or 6/6) is a measurement of visual acuity and means that the vision is perfect. If a person has a visual acuity of 20/40, it means he is able to see an object from a distance of 20 feet, while a normal person can see the same object from a distance of 40 feet. In other words, he has a slightly poorer vision than the normal person. It is possible to have vision superior to 20/20: the maximum acuity of the human eye without visual aids (such as binoculars) is generally thought to be around 20/10.
The difference in the visual acuity measurement used (i.e. 20/20 vs 6/6) is due to the variance in unit measurements used in UK and US. In UK, "metres" is used as the unit measurement, whereas in US, "feet" is used.