Corrective lens require correction of a refractive error. Some people with good natural eyesight like to wear eyeglasses as a style accessory, or want to change the appearance of their eyes using novelty contact lenses. Although corrective lenses can be produced in many different profiles, the most common is ophthalmic or convex-concave. In an ophthalmic lens, both the front and back surface have a positive radius, resulting in a positive / convergent front surface and a negative / divergent back surface. The difference in curvature between the front and rear surface leads to the corrective power of the lens. In hyperopia a
Focal cloud Focal cloud A focal cloud is the collection of focal points of an "imperfect" lens or parabolic reflector whether optical, electrostatic or electromagnetic. This includes parabolic antennas and lens-type reflective antennas of all kinds. The effect is analogous to the circle of confusion in photography. In a perfect lens or parabolic reflector, rays parallel to the device's axis striking the lens or reflector all pass through a single point, the focal point. In an imperfectly constructed lens or reflector, rays passing through different parts of the element do not converge to a single point but have different focal points. The