Presbyopia is the difficulty to see close objects clearly, linked to a loss of accommodative amplitude in the eye.
Presbyopia is the inibility to focus on close objects due to the loss of accommodation in the eye.
The young eye has the ability to adapt, allowing it to see all distances, near and far. This is thanks to the crystallin; a supple lense which modifies itself and changes shape to maintain clear vision.
From the age of 45 the crystallin looses its flexibility and near vision becomes difficult and eventually impossible.
Presbyopia adds itself to any existing optical conditions (myopia, hypermetropia, astigmatism), therefore making correction neccesary for both near and far vision.
It tends to stabalise around the age of 60.