Keratoconus is a common condition in which the cornea becomes thinned and weaker, resulting in a bulged shape, poor quality light focusing and blurred distorted vision.
Presbyopia refers to the natural age related decline of reading vision, which starts to occur in the forties.
Astigmatism occurs when polarized light travelling in a horizontal plane is focused differently to light travelling in a vertical plane.
Eyes which are long sighted are smaller than average so light is (in theory) focused at a point behind the back of the eye.
Shortsighted people have poor distance vision but good near vision, and this near vision stimulation in childhood leads to the development of good vision in both eyes.
Cataracts are the commonest eye problem requiring surgery. They occur when the clear natural lens inside the eye becomes cloudy and is usually as part of the ageing process.