ABSTRACT
The main purpose of an ophthalmic lens is to correct ammetropies, like myopia and hypermetropia, frequently combined with astigmatism, as illustrated in Figure 10.2. They are also used to compensate for deficiencies in the accommodation to observe objects close to the eye. These lenses have the shape of a thin meniscus lens placed in front of the eye, as shown in Figure 10.3. The lens is mounted in a frame that fixes the lens position at a short distance from the eye, such that the vertex of the concave surface of the lens is 14 mm in front of the cornea. The lens forms a virtual image of the object being observed, at the proper distance from the eye to observe it clearly focused. 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. The power of the lens in diopters is the inverse of the back focal length, measured in meters. According to the American Optometric Association Standard, the tolerance in the power of an ophthalmic lens is ±0.06 diopters. The International Organization for Standardization (ISO) has a complete list of norms for spectacle lenses (ISO 8980-1). They have specified a more relaxed power tolerance that is different for every lens power, where the highest the diopter power, the larger the tolerance can be.
