Angenieux retrofocus
The Angénieux retrofocus photographic lens
Photographic lens
A camera lens is an optical lens or assembly of lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capable of storing an image chemically or electronically.While in principle a simple convex lens will suffice, in...

 is a wide-angle lens
Wide-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,...

 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
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

 in 1950 by Pierre Angénieux
Pierre Angénieux
Pierre Angénieux was a French engineer and optician, one of the inventors of the modern zoom lenses, and famous for introducing the Angénieux retrofocus.-Biography:...


The 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
Image plane
In 3D computer graphics, the image plane is that plane in the world which is identified with the plane of the monitor. If one makes the analogy of taking a photograph to rendering a 3D image, the surface of the film is the image plane. In this case, the viewing transformation is a projection that...

) to a figure 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 for practical, not optical reasons, because telephotos can then 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.

The inverted telephoto design was employed in the early Technicolor
Technicolor is a color motion picture process invented in 1916 and improved over several decades.It was the second major process, after Britain's Kinemacolor, and the most widely used color process in Hollywood from 1922 to 1952...

 "3-strip" cameras since the beam splitter
Beam splitter
A beam splitter is an optical device that splits a beam of light in two. It is the crucial part of most interferometers.In its most common form, a rectangle, it is made from two triangular glass prisms which are glued together at their base using Canada balsam...

 unit behind the lens took up a significant amount of space, so that a long back focal distance was essential. This 1931 design was by H. W. Lee of Taylor, Taylor and Hobson (British patent no. 355,452). Also, wide-angle lenses for narrow-gauge movie cameras had to be of this type.

In still photography, a single-lens reflex
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...

 camera requires a space for the reflex mirror, imposing a limit on the use of wide-angle lenses of normal designs. The retrofocus lenses addressed this situation by shifting the focal plane further back than where it would normally be, 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 of a non-retrofocal lens would be in the way as the mirror flipped up and down during exposure.

Made in focal lengths of 24 mm, 28 mm, and 35 mm, the Angénieux retrofocus lens inspired all 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 20th century.
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