Saturday, November 17, 2012

How a Lens Works


At its very simplest, a lens element is a (mostly) transparent element made of glass, plastic, or some other material, with one or two curved surfaces that bend (refract) the direction of light as it passes through. A curve that bulges outward (convex) causes the light to converge towards a single point of focus. A curve that bulges inward (concave) causes the light to diverge, instead.
One of the surfaces of the lens element can be flat (planar) and not curved. Figure 1.1 shows a simple lens element with two convex surfaces, while in Figure 1.2, you can see an equally simple lens element with two concave surfaces. Lens elements can be built with any combination of convex, concave, and planar surfaces (see Figure 1.3) depending on the desired properties of the lens to magnify, reduce, or correct for defects called aberrations. Most lens elements have spherical curvature; that is, the curve of the element is the same as the surface of a sphere of appropriate size. Some lens elements have more complex, non-spherical surfaces that can provide more sophisticated optical corrections, and are called aspherical. Individual lens elements can be created by grinding and polishing a piece of lens material to the desired shape, or by molding. Photographic lenses usually consist of three or more lens elements arranged in one or more groups that provide the desired optical effects, such as a telephoto or wide-angle viewpoint. Figure 1.4 shows a lens design that has been used for more than 100 years, called a Cooke triplet, that is notable because it was the first photographic lens to provide sharpness right out to the edge of the image. More complex designs followed, such as the four-element Tessar patented by the Zeiss optical company, shown in Figure 1.5.

Lenses are designated by their focal length, which is the distance from the optical center of the lens to its point of convergence or focus at the sensor (or, in a film camera, at the film plane). The more strongly a lens converges light, the shorter its focal length, and the wider its angle of view; a lens that converges light over a greater distance provides a magnification effect, and is most often called a telephoto lens. (Technically, a telephoto lens is actually one particular type of long-focal-length lens.)  Some lenses, including zoom lenses, have moving elements that can be shifted to change the focal length of the lens, thus altering its field of view and magnification. (See Figure 1.6.)






Source : David Busch's Digital SLR Lens

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