Astigmatism occurs when the cornea and/or lens start to grow into the shape of a cylinder and not a sphere.

Hyperopia occurs when the cornea and the lens focus light behind the retina.

Presbyopia occurs when the intra-ocular lens becomes sufficiently large and hard that its shape cannot be affected by its own eye muscles.

Myopia occurs when the cornea and the lens focus light before the retina (the back surface of the inside of the eye responsible for sensing

Prescription eye glasses are designed to help the eyes focus light on the retina.

Refractive eye surgery refers to a collection of surgeries that change the power of either the cornea or the lens.