Angenieux retrofocus

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The Angenieux 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. The Angenieux retrofocus was invented in France in 1950 by Pierre Angénieux.

The telephoto lens configuration combines positive and negative lens groups with the positive at the front, so as to make the lens physically shorter than its focal length. The inverted telephoto configuration does the reverse, employing one or more negative lens groups at the front to make the lens longer than its focal length. The reason is to increase the back focal distance of the lens, the distance between the back of the lens and the image plane, to allow for additional optical or mechanical parts to fit between them.

The inverted telephoto design was employed in early Technicolor cameras since the beam splitter unit behind the lens took up much space, so lenses with wider angle of view needed to be of inverted telephoto configuration. Also, wide-angle lenses for narrow-gauge movie cameras had to be of this type. Since the 35mm single-lens reflex (SLR) requires a space for the reflex mirror, it imposes a limit on the use of wide-angle lenses of normal designs, unless the reflex mirror is locked in the up position. The Retrofocus lenses rectified this situation by making wider lenses usable while retaining normal viewing and focussing.

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 single-lens reflex, and helped to make it the definitive camera type of the modern period.