Construction of Simple Lenses
Most lenses are spherical lenses: their two surfaces are parts of the surfaces of spheres, with the lens axis ideally perpendicular to both surfaces. Each surface can be convex (bulging outwards from the lens), concave (depressed into the lens), or planar (flat). The line joining the centres of the spheres making up the lens surfaces is called the axis of the lens. Typically the lens axis passes through the physical centre of the lens, because of the way they are manufactured. Lenses may be cut or ground after manufacturing to give them a different shape or size. The lens axis may then not pass through the physical centre of the lens.
Toric or sphero-cylindrical lenses have surfaces with two different radii of curvature in two orthogonal planes. They have a different focal power in different meridians. This is a form of deliberate astigmatism.
More complex are aspheric lenses. These are lenses where one or both surfaces have a shape that is neither spherical nor cylindrical. Such lenses can produce images with much less aberration than standard simple lenses. These in turn evolved into freeform (digital/adaptive/corrected curve) spectacle lenses, where up to 20,000 ray paths are calculated from the eye to the image taking into account the position of the eye and the differing back vertex distance of the lens surface and its pantoscopic tilt and face form angle. The lens surface(s) are digitally adapted at nanometer levels (normally by a diamond stylus) to eliminate spherical aberration, coma and oblique astigmatism. This type of lens design almost completely fulfills the sagittal and tangential image shell requirements first described by Tscherning in 1925 and further described by Wollaston and Ostwalt. These advanced designs of spectacle lens can improve the visual performance by up to 70% particularly in the periphery.
Read more about this topic: Lens (optics)
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