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, and super telephoto: lenses covering between 8° through less than 1° field of
view.
Construction
In contrast to a telephoto lens, for any given focal length a simple lens of non-telephoto
design is constructed from one lens. To focus on an object at infinity, the distance from
this single lens to focal plane of the camera 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. While the
optical centre of a simple lens is within the construction, the telephoto lens moves
the optical centre in front of the construction.
A telephoto lens works by having the outermost 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 curved 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 non-Catadioptric telephoto lens
for civilian use was made by Carl Zeiss and has a focal length of 1700 mm with a maximum
aperture of f/4, implying a 425 mm entrance pupil. It is designed for use with a medium
format Hasselblad 203 FE camera and weighs 256 kg.
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. History
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 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 in 1890. See also
Afocal photography Film format
Secret photography Photographic lens design
Barlow lens Zoom lens
References
External links Information on Catadioptric mirror lenses
Further clarification: Why Telephoto? 3 part series on Cheap Super Telephoto Lenses
Stanford University CS 178 interactive Flash applet showing how a telephoto zoom lens works.