The eye can be compared to a camera. Light passes through the cornea, which begins the process of bending (refracting) the light rays. It then passes through the pupil, which is a round opening in the centre of the iris beyond which it travels through the lens. The lens bends the light rays further in order to focus them on the retina.
If the image is not focused, the retina receives a blurred image, and the eye is said to have a ‘refractive error’. The majority of adults in the UK wear glasses or contact lenses to correct such errors, which include short-sight, long-sight and astigmatism.
Another reason people need to wear glasses, typically over the age of 40, is to help reading in order to counteract presbyopia.
Many patients are used to wearing their glasses and do not want to stop. However, for those who wish to reduce their dependency on spectacles or contact lenses, techniques have improved such that surgery is a safer and better option than ever before.
Typically, the eyeball is too long, causing light to be focused in front of the retina and resulting in a blurred image of distant objects. However, close objects are seen clearly and short-sighted people can read without glasses. 
Hyperopia, or long-sightedness, is the opposite of myopia. Distant objects are relatively clear, but close-up objects appear blurred. The eyeball is too short, causing the light rays to focus behind the retina. 
This term describes an irregularity in the shape of the cornea. A normally seeing eye has a cornea shaped like a football. An astigmatic eye is shaped more like a rugby ball. As a result, the visual image is distorted.
This is a normal ageing process and results in blurring of near vision. The muscle which allows the lens of the eye to change shape gradually
loses its power, causing an inability to focus light rays from nearby objects. Hence, most people in their 40s find they need reading glasses.