Retrofocus
H is the main plane on the image side
The term retrofocus describes a special design of lenses with a short focal length . The word retrofocus describes the essential characteristic of these lenses: it is derived from the Latin retro : backwards, backwards and focus : fireplace, stove , in the figurative sense focal point . Translated, retrofocus means: reset the focus .
motivation
When setting a lens to the distance "infinite" , the distance between the main plane of the lens and the image plane corresponds to the focal length . If the objective consists of a single thin converging lens , this would lie exactly in the main plane on the image side. In the case of lenses with a very short focal length, however, this distance can become too small for certain technical applications. For example, with a single -lens reflex camera, there must still be enough space between the lens and the image plane for the oscillating mirror . The retro-focus construction allows the focal distance to zoom the lens without changing the focal length. The lenses of the objective can lie in front of the main plane on the image side. Lenses with focal lengths that are significantly smaller than the minimum possible back focal length of the camera system ( around 38 mm for 35mm single lens reflex cameras, depending on the system) are usually manufactured in retrofocus design.
history
The first retrofocus lenses were developed for color film cameras in 1931 . These early cameras had to leave enough space behind the lens for a color divider . As a result, the distance between the lens and the film plane became so great that conventional lenses could no longer be used for smaller focal lengths. The first retrofocus lens for 35mm cameras was developed by Pierre Angénieux in 1950 .
In the first designs, a divergent lens was placed in front of an existing lens, which deteriorated the imaging performance. The construction of a high-quality retrofocus lens is complex, but manageable using modern calculation and manufacturing methods. The asymmetrical design and the large front lenses required for large image angles make it difficult to correct the aberrations . In order to achieve good imaging performance even at close range, lens elements are shifted against each other when focusing in high-quality constructions ( floating elements ).
Retrofocus lens / telephoto lens
The retro-focus construction is the reverse of Tele shape results (English: telephoto design ) lenses: telephoto lenses are shorter than its focal length. With the telephoto lens, there is first a positive group (converging lens) in the beam path, followed by a negative group (diverging lens), whereby the overall length is shorter than the focal length ( principle of the Galilei telescope ). With retrofocus lenses, the order is reversed, which increases the overall length.
In photographic practice it has become common to use the term telephoto lens in general for lenses with a focal length greater than the normal focal length - even if it is not a real “telephoto” construction.
See also
- Distagon , type and brand name for retrofocus lenses from Carl Zeiss AG , Oberkochen
Web links
- Leica R lenses page 3, section retrofocus (PDF file; 1.02 MB)
- Natural Vignetting (English)
Individual evidence
- ↑ Zeiss website for the Distagon 2.8 / 15 mm , accessed August 3, 2015
- ↑ Data sheet for the Planar 1.4 / 50 mm with Contax / Yashica connection PDF, accessed August 3, 2015