The Angénieux retrofocus photographic lens is a

wide-angle lens In photography and cinematography, a wide-angle lens refers to a lens whose focal length is substantially smaller than the focal length of a normal lens for a given film plane. This type of lens allows more of the scene to be included in the ph ...

design that uses an inverted telephoto configuration. The popularity of this lens design made the name retrofocus synonymous with this type of lens. The Angénieux retrofocus for still cameras was introduced in France in 1950 by Pierre Angénieux.

Inverted telephoto concept

The

telephoto A telephoto lens, in photography and cinematography, 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 ...

lens configuration combines positive and negative lens groups with the positive at the front, so as to reduce the back focal distance of the lens (the distance between the back of the lens and the image plane) to a figure shorter than the focal length. This is for practical, not optical reasons, because it allows telephoto lenses to be made shorter and less cumbersome. The inverted telephoto configuration does the reverse, employing one or more negative lens groups at the front to increase the back focal distance of the lens – possibly to a figure greater than the focal length – in order to allow for additional optical or mechanical parts to fit behind the lens. Lee - Inverted telephoto (1930)

The inverted telephoto design was first employed in the 1930s by

Taylor-Hobson Taylor Hobson is an English company founded in 1886 and located in Leicester, England. Originally a manufacturer of still camera and cine lenses, the company now manufactures precision metrology instruments—in particular, profilometers for the ...

for the early Technicolor "3-strip" cameras since the

beam splitter A beam splitter or ''beamsplitter'' is an optical device that splits a beam of light into a transmitted and a reflected beam. It is a crucial part of many optical experimental and measurement systems, such as interferometers, also finding wide ...

unit behind the lens required significant space, so that a long back focal distance was essential. Horace Lee patented an inverted telephoto lens design in 1930 with an

angle of view The angle of view is the decisive variable for the visual perception of the size or projection of the size of an object. Angle of view and perception of size The perceived size of an object depends on the size of the image projected onto the ...

of 50° and maximum aperture of 2 which afforded a distance between the rear element and the film plane approximately 10% greater than the focal length. Joseph Ball showed how a beam-splitting apparatus could be fitted in the space gained. Also, wide-angle lenses for narrow-gauge movie cameras had to be of this type because of the shutter mechanism that had to fit in between. In still photography, a single-lens reflex camera requires a space for the reflex mirror, imposing a limit on the use of wide-angle lenses of symmetric designs. The retrofocus lens addressed this situation by increasing the distance between the rear element and the focal plane, thus making wider-angle lenses usable while retaining normal viewing and focusing. Unless the reflex mirror were locked in the "up" position, blacking out the viewfinder, the rearmost element(s) of a non-retrofocus (symmetric wide-angle) lens would interfere with the movement of the mirror as it flipped up and down during exposure.

Implementation

Pierre Angénieux applied for a patent in 1950. In the original patent, he presented two lenses with an

of 65°, approximately equal to the view of a f=35mm lens on the

35mm format 135 film, more popularly referred to as 35 mm film or 35 mm, is a format of photographic film used for still photography. It is a film with a film gauge of loaded into a standardized type of magazine – also referred to as a cass ...

for still cameras; the first example had a maximum aperture of 2.5, while the second example had a maximum aperture of 2.2. At approximately the same time, Harry Zöllner and Rudolf Solisch applied for a similar patent on an inverted telephoto lens design, branded Flektogon, for Carl Zeiss Jena. Carl Zeiss Oberkochen also created an inverted telephoto design branded Distagon (5.6/60mm) for the Hasselblad 1000F in 1952. Made in focal lengths of 24 mm, 28 mm, and 35 mm, the Angénieux retrofocus lens inspired other lens makers to produce wide-angle lenses of this type for almost every 35mm SLR, and helped to make it the definitive camera type of the late 20th century.

References

Bibliography

* * {{DEFAULTSORT:Angenieux retrofocus Photographic lens designs