Angénieux retrofocus

The term retro-focus refers to a special construction of lenses with short focal length. The word retro focus here describes the genes of the essential property of these lenses: it derives from the Latin retro: backward, back and focus: fireplace, stove, figuratively focal point. Retro Focus translates thus: Reset the focal point.

Motivation

When setting a lens to the distance "infinity" corresponds to the distance between the image-side principal plane of the lens and the image plane just the focal length. When the lens is composed of a single thin lens group, that would just lie in the image-side principal plane. For lenses with very short focal length, however, this distance for certain technical applications can be too small. For example, still has enough space for the oscillating mirror stay with a single lens reflex camera between the lens and the image plane. The retro-focus design allows it to increase the average length of the lens without changing the focal length. The lens can here are the image-side principal plane. For small SLRs you made ​​from the above-mentioned basic lenses with focal lengths of 40 mm and less commonly in retro-focus construction.

History

The first retro-focus lenses were developed in 1931 for color film cameras. Behind the lens had to stay in these early cameras enough space for a color splitter. Thus, the distance between the lens and film plane was so great that for smaller focal lengths the conventional lenses could not be used. The first retro-focus lens for miniature cameras was developed in 1950 by Pierre Angenieux.

In the first designs, a diverging lens was placed in front of an existing lens, which the imaging performance deteriorated. The construction of a high quality retro-focus lens is expensive, but manageable by modern calculation and production methods. The asymmetric design and the necessary for wide angle large front lenses make the correction of aberrations difficult. In order to achieve good imaging performance even at close range, the lens elements are in high quality designs shifted against each other when focusing (floating elements ).

Retro-focus lens / telephoto lens

The retro-focus construction is the inverse of Tele- design (English: telephoto design) lenses: telephoto lenses are shorter than its focal length. At the telephoto lens is a positive group ( positive lens ) only in the beam path, followed by a negative group ( diverging lens ), making the overall length is shorter than the focal length (the principle of Galilean telescope ). In retro-focus lenses, the order is reversed, thereby increasing the overall length.

In the photographic practice, it has become common to use the term generally telephoto lenses with a longer focal length than the normal focal length - even if it's not a real " telephoto " design.