Телеобъективы: конструкция, типы и применение.

Type of camera lens with long focal length

A telephoto lens, also known as telelens, is a specific type of a long focus lens used in photography and cinematography, in which the physical length of the lens is shorter than the focal length. This is achieved by incorporating a special lens group known as a telephoto group that extends the light path to create a long focus lens in a much shorter overall design. The angle of view and other effects of long focus lenses are the same for telephoto lenses of the same specified focal length. Long focal length lenses are often informally referred to as telephoto lenses, although this is technically incorrect: a telephoto lens specifically incorporates the telephoto group.

In contrast to a telephoto lens, for any given focal length a simple lens of non telephoto design is constructed from one lens (which can, to minimize aberrations, consist of several elements to form an achromatic lens). 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 this focal length. 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. 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, at least once in the case of the Olympus XA where it permitted a 35 mm focal length to fit in an extra compact camera body.

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.

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). +Typical telephoto lens focal lengths (mm) for different formats Short / Portrait Medium Super Angle of view (diag.) 34–18° 18–8° 8–1°   1" – – – 4/3 – – – APS C – – – 35 mm 70–135 135–300 300–2000 6×6 (120 film) – – – 4×5 (large format) – – –

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.