Telephoto lens
Encyclopedia
In photography
and cinematography
, a telephoto lens is a specific type of a long-focus lens
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.
Telephoto lenses are sometimes broken into the further sub-types of medium telephoto: lenses covering between a 30° and 10° field of view (85mm to 135mm in 35mm film format), and super telephoto: lenses covering between 8° through less than 1° field of view (over 300mm in 35mm film format).
If a camera
lens were to be constructed from a single lens of 500 mm focal length, then when the lens is focused on an object at infinity, the lens will be 500 mm away from the focal plane where the film or sensor is. The centre of the lens is referred to as the optical centre of the lens. Even constructing the lens out of several elements to minimize aberrations
, will still have the optical centre within the construction.
As the focal length of such lenses increases, the physical length of lens becomes inconveniently long. But such lenses are not telephoto lenses, no matter how extreme the focal length. They are simply known as long-focus lenses. A telephoto lens works by having the outermost (i.e. light gathering) element of a much shorter focal length than the equivalent long-focus lens and then incorporating a second set of elements close to the film or sensor plane that extend the cone of light so that it appears to have come from a lens of much greater focal length.
The basic construction of a telephoto lens consists of front lens elements that, as a group, have a positive focus. The focal length of this group is shorter than the effective focal length of the lens. The converging rays from this group are intercepted by the rear lens group, sometimes called the "telephoto group," which has a negative focus. The simplest telephoto designs could consist of one element in each group, but in practice, more than one element is used in each group to correct for various aberrations. The combination of these two groups produces a lens assembly that is physically shorter than a long-focus lens producing the same image size.
This same property is achieved in camera lenses that combined mirrors with lenses. These designs called catadioptric, 'reflex', or 'mirror' lenses have a curved mirror
as the primary objective with some form of negative lens in front of the mirror to correct optical aberrations. They also use a secondary mirror
to relay the image that extends the light cone the same way the negative lens telephoto group does. The mirrors also fold the light path. This makes them much shorter, lighter, and cheaper than an all refractive lens, but at the cost of some optical compromises due to aberrations caused by the central obstruction from the secondary mirror.
The heaviest telephoto lens was made by Carl Zeiss
and has a focal length of 1700 mm with a maximum aperture
of , implying a 425 mm (16.7 inch) entrance pupil
. It is designed for use with a medium format Hasselblad
203 FE camera and weighs 256 kg (564 lb).
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 lens
es that are telephotos at one extreme of the zoom range and retrofocus at the other are now common.
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 the Russian warship Vjestnik 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 Vjestnik 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.
Photography
Photography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a light-sensitive material such as photographic film...
and cinematography
Cinematography
Cinematography is the making of lighting and camera choices when recording photographic images for cinema. It is closely related to the art of still photography...
, a telephoto lens is a specific type of a long-focus lens
Long-focus lens
In photography, a long-focus lens is a camera lens which has a focal length that is longer than the diagonal measure of the film or sensor that receives its image....
in which the physical length of the lens is shorter than the focal length
Focal length
The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus...
. 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
Angle of view
In photography, angle of view describes the angular extent of a given scene that is imaged by a camera. It is used interchangeably with the more general term field 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.
Telephoto lenses are sometimes broken into the further sub-types of medium telephoto: lenses covering between a 30° and 10° field of view (85mm to 135mm in 35mm film format), and super telephoto: lenses covering between 8° through less than 1° field of view (over 300mm in 35mm film format).
Construction
If a camera
Camera
A camera is a device that records and stores images. These images may be still photographs or moving images such as videos or movies. The term camera comes from the camera obscura , an early mechanism for projecting images...
lens were to be constructed from a single lens of 500 mm focal length, then when the lens is focused on an object at infinity, the lens will be 500 mm away from the focal plane where the film or sensor is. The centre of the lens is referred to as the optical centre of the lens. Even constructing the lens out of several elements to minimize aberrations
Aberration in optical systems
Aberrations are departures of the performance of an optical system from the predictions of paraxial optics. Aberration leads to blurring of the image produced by an image-forming optical system. It occurs when light from one point of an object after transmission through the system does not converge...
, will still have the optical centre within the construction.
As the focal length of such lenses increases, the physical length of lens becomes inconveniently long. But such lenses are not telephoto lenses, no matter how extreme the focal length. They are simply known as long-focus lenses. A telephoto lens works by having the outermost (i.e. light gathering) element of a much shorter focal length than the equivalent long-focus lens and then incorporating a second set of elements close to the film or sensor plane that extend the cone of light so that it appears to have come from a lens of much greater focal length.
The basic construction of a telephoto lens consists of front lens elements that, as a group, have a positive focus. The focal length of this group is shorter than the effective focal length of the lens. The converging rays from this group are intercepted by the rear lens group, sometimes called the "telephoto group," which has a negative focus. The simplest telephoto designs could consist of one element in each group, but in practice, more than one element is used in each group to correct for various aberrations. The combination of these two groups produces a lens assembly that is physically shorter than a long-focus lens producing the same image size.
This same property is achieved in camera lenses that combined mirrors with lenses. These designs called catadioptric, 'reflex', or 'mirror' lenses have a curved mirror
Curved mirror
A curved mirror is a mirror with a curved reflective surface, which may be either convex or concave . Most curved mirrors have surfaces that are shaped like part of a sphere, but other shapes are sometimes used in optical devices...
as the primary objective with some form of negative lens in front of the mirror to correct optical aberrations. They also use a secondary mirror
Secondary mirror
A secondary mirror is the second deflecting or focusing mirror element in a reflecting telescope. Light gathered by the primary mirror is directed towards a focal point typically past the location of the secondary. Secondary mirrors in the form of an optically flat diagonal mirror are used to...
to relay the image that extends the light cone the same way the negative lens telephoto group does. The mirrors also fold the light path. This makes them much shorter, lighter, and cheaper than an all refractive lens, but at the cost of some optical compromises due to aberrations caused by the central obstruction from the secondary mirror.
The heaviest telephoto lens was made by Carl Zeiss
Carl Zeiss
Carl Zeiss was a German maker of optical instruments commonly known for the company he founded, Carl Zeiss Jena . Zeiss made contributions to lens manufacturing that have aided the modern production of lenses...
and has a focal length of 1700 mm with a maximum aperture
Aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...
of , implying a 425 mm (16.7 inch) entrance pupil
Entrance pupil
In an optical system, the entrance pupil is the optical image of the physical aperture stop, as 'seen' through the front of the lens system. The corresponding image of the aperture as seen through the back of the lens system is called the exit pupil...
. It is designed for use with a medium format Hasselblad
Hasselblad
Victor Hasselblad AB is a Swedish manufacturer of medium-format cameras and photographic equipment based in Gothenburg, Sweden.The company is best known for the medium-format cameras it has produced since World War II....
203 FE camera and weighs 256 kg (564 lb).
Retrofocus lenses
Inverting the telephoto configuration, employing one or more negative lens groups in front of a positive lens group, creates a wide-angle lensWide-angle lens
From a design perspective, a wide angle lens is one that projects a substantially larger image circle than would be typical for a standard design lens of the same focal length; this enables either large tilt & shift movements with a view camera, or lenses with wide fields of view.More informally,...
with an increased back focal distance. These are called retrofocus lenses
Angenieux retrofocus
The Angénieux retrofocus photographic lens is a wide-angle lens design that uses an inverted telephoto configuration. The popularity of this lens design made the name retrofocus synonymous with this type of lens...
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
Single-lens reflex camera
A single-lens reflex camera is a camera that typically uses a semi-automatic moving mirror system that permits the photographer to see exactly what will be captured by the film or digital imaging system, as opposed to pre-SLR cameras where the view through the viewfinder could be significantly...
. Zoom lens
Zoom lens
A zoom lens is a mechanical assembly of lens elements for which the focal length can be varied, as opposed to a fixed focal length lens...
es that are telephotos at one extreme of the zoom range and retrofocus at the other are now common.
History
The concept of the telephoto lens, in reflecting form, was first described by Johannes KeplerJohannes Kepler
Johannes Kepler was a German mathematician, astronomer and astrologer. A key figure in the 17th century scientific revolution, he is best known for his eponymous laws of planetary motion, codified by later astronomers, based on his works Astronomia nova, Harmonices Mundi, and Epitome of Copernican...
in his Dioptrice of 1611, and re-invented by Peter Barlow in 1834.
Histories of photography usually credit Thomas Rudolphus Dallmeyer
Thomas Rudolphus Dallmeyer
Thomas Rudolphus Dallmeyer , English optician, was the son of John Henry Dallmeyer who ran an optics business.He assumed control of the business on the failure of his father's health, was principally known as the first to introduce the telephoto lens into ordinary practice , and he was the author...
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
John Henry Dallmeyer
John Henry Dallmeyer , Anglo-German optician, was born at Loxten, Westphalia, the son of a landowner.On leaving school at the age of sixteen he was apprenticed to an Osnabrück optician, and in 1851 he came to London, where he obtained work with an optician, W Hewitt, who shortly afterwards, with...
in 1860.
In 1883 or 1884 New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
photographer Alexander McKay discovered he could create a much more manageable long-focus lens by combining a shorter focal length telescope objective
Objective (optics)
In an optical instrument, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image. Objectives can be single lenses or mirrors, or combinations of several optical elements. They are used in microscopes, telescopes,...
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
Wellington
Wellington is the capital city and third most populous urban area of New Zealand, although it is likely to have surpassed Christchurch due to the exodus following the Canterbury Earthquake. It is at the southwestern tip of the North Island, between Cook Strait and the Rimutaka Range...
, and two of these can be unequivocally dated as having been taken during May 1886. One of McKay’s photographs shows the Russian warship Vjestnik 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 Vjestnik 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.
See also
- Afocal photographyAfocal photographyAfocal photography, also called afocal imaging or afocal projection is a method of photography where the camera with its lens attached is mounted over the eyepiece of another image forming system such as a optical telescope or optical microscope, with the camera lens taking the place of the human...
- Film formatFilm formatA film format is a technical definition of a set of standard characteristics regarding image capture on photographic film, for either stills or movies. It can also apply to projected film, either slides or movies. The primary characteristic of a film format is its size and shape.In the case of...
- Secret photographySecret photographySecret photography refers to the use of an image or video recording device to photograph or film a person who is unaware that they are being intentionally photographed or filmed...
- Photographic lens designPhotographic lens designThe design of photographic lenses for use in still or cine cameras is intended to produce a lens that yields the most acceptable rendition of the subject being photographed within a range of constraints that include cost, weight and materials...
- Barlow lensBarlow lensThe Barlow lens, named for its creator, the English engineer Peter Barlow, is a diverging lens which, used in series with other optics in an optical system, increases the effective focal ratio of an optical system as perceived by all components after it in the system...
- Zoom lensZoom lensA zoom lens is a mechanical assembly of lens elements for which the focal length can be varied, as opposed to a fixed focal length lens...
External links
- Information on Catadioptric mirror lenses
- Further clarification: Why Telephoto?
- 3 part series on Cheap Super Telephoto Lenses (300-500mm)
- Stanford University CS 178 interactive Flash applet showing how a telephoto zoom lens works.