Anatomy of the Eye
Visual Disorders

The human eye is like a camera. The cornea and the crystalline lens of a healty eye work like a camera lens that drives the incident light exactly on the retina.

In visual defects the force of refraction and the focal length don't armonize to each other, which should be indispensable for a right focusing. It happens because the lens threw an image in a wrong way in front of or behind the retina.