Telephoto lens

A simple photographic lens may be constructed using one lens element of a given focal length; to focus on an object at infinity, the distance from this single lens to focal plane of the camera (where the sensor or film is) has to be adjusted to the focal length of that lens. For example, given a focal length of 500 mm, the distance between lens and focal plane is 500 mm. The farther the focal length is increased, the more the physical length of such a simple lens makes it unwieldy. In practice, to minimize optical aberrations, instead of a single lens element, these simple lenses usually are constructed using several elements to form an achromatic lens. But such simple lenses are not telephoto lenses, no matter how extreme the focal length – they are known as long-focus lenses. The telephoto lens design has also been used for wide angles; in the case of the Olympus XA, the telephoto arrangement permitted a 35 mm focal length to fit in an extra compact camera body. Retrofocus lenses Inverting the telephoto configuration, employing one or more negative lens groups in front of a positive lens group, creates a wide-angle lens with an increased back focal distance. These are called retrofocus lenses or inverted telephotos, which have greater clearance from the rear element to the film plane than their focal length would permit with a conventional wide-angle lens optical design. This allows for greater clearance for other optical or mechanical parts such as the mirror parts in a single-lens reflex camera. Zoom lenses that are telephotos at one extreme of the zoom range and retrofocus at the other are now common. ==Naming==

Telephoto lenses are sometimes divided into the further sub-types of short or portrait (85–135 mm in 35 mm film format), medium (135–300 mm in 35 mm film format) and super (over 300 mm in 35 mm film format). ==History==

(1981), a 35 mm camera with a telephoto zoom lens with 70-210 mm focal length. like the Panasonic Lumix DMC-TZ18 (2010) have superzoom lenses with a large range of focal lengths. The lens is completely stored inside the camera in switched-off state and has a maximum focal length (shown) of 384 mm (calculated equivalent to 35 mm film), minimum is 24 mm, a zoom factor of 16×. The concept of the telephoto lens, in reflecting form, was first described by Johannes Kepler in his Dioptrice of 1611, and re-invented by Peter Barlow in 1834. Histories of photography usually credit Thomas Rudolphus Dallmeyer with the invention of the photographic telephoto lens in 1891, though it was independently invented by others about the same time; some credit his father John Henry Dallmeyer in 1860. In 1883 or 1884, New Zealand photographer Alexander McKay discovered he could create a much more manageable long-focus lens by combining a shorter focal length telescope objective lens with negative lenses and other optical parts from opera glasses to modify the light cone. Some of his photographs are preserved in the holdings of the Turnbull Library in Wellington, and two of these can be unequivocally dated as having been taken during May 1886. One of McKay's photographs shows a warship anchored in Wellington harbour about two and a half kilometres away, with its rigging lines and gun ports clearly visible. The other, taken from the same point, is of a local hotel, the Shepherds Arms, about 100 metres distant from the camera. The masts of the ship are visible in the background. McKay's other photographic achievements include photo-micrographs, and a ‘shadow-less technique’ for photographing fossils. McKay presented his work to the Wellington Philosophical Society (the precursor of the Royal Society of New Zealand) in 1890. Starting in the mid-1970s, Japanese manufacturers introduced telephoto lenses which focused by moving the smaller (diverging) rear group, rather than moving the entire optical system as a unit; in some cases, a second converging group was added behind the diverging group. This was marketed as internal focusing, differential focusing, or rear focusing and the concept was derived from zoom lens designs. ==See also==