Whenever you see through your SLR's viewfinder, or on your digital point-and-shoot's LCD screen, you are using one of humanity's oldest inventions: the lens.
This last week, we've been very busy, focusing on photography and it's history, as well as the functioning of a typical SLR. We're now closing with the unknown basics of photography: how lenses work.
A lens is an optical device that transmits, refracts, and concentrates light, as well as having axial simmetry, that is, it is symmetric around an axis, so if you were to rotate the object based on that axis, the appearance and form of the object would result unchanged. Lenses are divided into two categories: Simple lenses and Compound Lenses.
A simple lens is formed by a single optical element, while a complex lens is formed by many different optical elements put together.
The oldest lens in the world, the Nimrud lens, is over three thousand years old, and comes from Iraq (formerly Persia, formerly Assyria). It was simply a piece of rock crystal which may have been used as a magnifying glass, or as a burning lens.
Lenses are regularly a portion of a sphere, with surfaces being convex, concave or planar.
When a lens has two convex surfaces, it is called a biconvex lens. A surface with a planar and a convex surface is named plano-convex. A lens with two concave surfaces is called biconcave, and a lens with a concave and a planar surface is called a plano-concave lens. There's also meniscus lens, which feature a convex and a concave surface.
Convex type lenses focus the light in a spot. Concave type lenses diverge light around an area. Meniscus lenses can be either positive or negative, depending on the thickness of the lens, and the difference of curvature between both lenses.