Refraction
Refraction is the process of determining the refractive (light bending) power of your eye. It involves the use of ophthalmic lenses to move the image of an object from either in front or behind the eye unto a specific point (retina) in the eye.
In refraction minus (concave) and plus (conves) ophthalmic lenses are used to cause the movement of the image from a point that causes blur to a preferred point on the retina. Minus lenses are used when the image formed is in front of the retina and plus lenses are used to move the image from behind the retina.