The cornea is the outer lens of the eye. It is a transparent lens that covers the iris and the pupil providing protection and the eye's general focusing power. The inside lens provides the fine focusing power. For clear vision, these two lenses work together to focus light rays precisely on the retina, which is the inner back layer of the eye. The retina gathers light and picks up visual images which are then transmitted to the brain. It is like the film of a camera.

Three elements contribute to an image being in focus:

the shape of the cornea, the power of the tens, and the length of the eyeball.

When all three elements are well coordinated, clear images are focused on the retina and normal vision results. However, due to hereditary and developmental factors, many eyes develop in such a way that one or more of these variables are imperfect, resulting in focussing problems.