Tan Tock Seng Hospital - Lasik Centre

A diopter is a unit of measurement that determines the refractive power of the eye. 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 slightly poorer vision than the normal person. Some people may have vision superior to 20/20: theoretically, the maximum acuity of the human eye is 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 difference in the unit of measurement used in UK and US. In UK, "metres" is used as the unit measurement, whereas in US, "feet" is used. The standard vision testing distance is usually 6 metres (or 20 feet).

- What Types of Refractive Errors Cause Poor Vision?

	Myopia (Short-sightedness or Near-sightedness) The myopic eye is longer than normal and distant images are focused in front of the retina instead of on it, causing a blurred image. Someone with myopia will see near objects clearly, but distant objects will be blur.

	Hyperopia (Long-sightedness or Far-sightedness) The hyperopic eye is shorter than normal and images are focused behind the retina, leading to a blurred image. Someone with hyperopia will see distant objects clearly but objects that are nearby will be blur.

	Astigmatism The astigmatic cornea is unevenly curved, resulting in blurred and multiple distorted images. This affects distant and near objects equally. Astigmatism may co-exist with myopia or hyperopia.

	Presbyopia (Old-sightedness) In presbyopia, the eye loses its ability to focus on near objects. It is caused by ageing and usually begins around 40 years of age. With time, presbyopia will progress and worsen, necessitating reading glasses. LASIK cannot prevent or correct presbyopia.