Ophthalmic lenses had been used to correct refraction defects in the eye since many centuries ago. The most important parameter in an ophthalmic lens is the back focal length, which is the distance from the vertex of the concave surface to the focus of the lens. Ophthalmic lenses were originally made out of glass, but most of them are made out of plastic. Most glass manufacturers produce ophthalmic glasses with many different characteristics. In hypermetropia, the opposite refractive defect, the eye is shorter than normal. An ophthalmic lens corrects this defect by shifting the real image formed by the optics of the eye to the correct position, a point in space conjugate to the retina. The image formed by the ophthalmic lens should also be at a constant distance for any object in the object surface. Thus, the image surface is also spherical, with the center of curvature at the stop.