As we get older, the lens of the eye thickens and slowly loses its flexibility making it difficult to focus on things close up. This loss of focusing ability is called Presbyopia. It is not a disease, but a normal change which affects everyone.
Presbyopia doesn’t occur suddenly and it doesn’t affect distance vision. It is a change which can not be prevented beginning in adolescence and is most noticeable around the age of 40 to 45, when vision at our normal reading distance becomes blurry.
The simplest form of correction is to wear a pair of reading glasses for close up work. Unfortunately this will cause blurred vision for long distances. Because of this, many people opt for bi-focal or multi-focal lenses to provide clear vision at both short and long distances.