How Do Lenses Help ?
The eye is a lens and focal point system. The focal length of the eye can be manipulated by flexing muscles in the eye socket to change the distance between the lens and the back of the eyeball. A near sighted, or even a far sighted, person has an eyeball that is too long or too short for the lens, and therefore the image produced by the eye's lens is focused either in front of or behind the back of the eye.
A lens, either in a pair of glasses or a contact lens, can assist the eye by effectively pre-focusing the image that enters the lens of the eye to a sharpness that can be accommodated by the focal length of the malformed eyeball, focusing the image sharply on the back of the eyeball.