The foci and interval are formed as though the surface were toric. Also referred to as Marginal Astigmatism . The point on a lens where rays of light are not displaced. It is the thinnest point on a minus lens and the thickest point on a plus lens. A lens that minimizes distortion is referred to as orthoscopic.
Orthoscopic lenses have very steep profiles.
Technically this is the angle formed by a plane perpendicular to the temple of a frame and the plane of the eye wires. Such a lens is free of mean oblique error without any consideration of oblique astigmatism . A lens that has a 1.25D base curve. The base curve is on the front surface of minus prescriptions and on the back on plus. It is a function of the index of the lens and the focal length.