Angénieux retrofocus

The telephoto lens configuration combines positive and negative lens groups with the negative at the rear, serving to magnify the image, which reduces 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 first practical telephoto lens was developed by Peter Barlow in the early 1800s, with the eponymous Barlow lens referring to the negative achromat inserted between the eye and a telescope. 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 negative front group also serves to increase peripheral illumination; some symmetric wide-angle lenses require a radially-graduated filter or other means to make the exposure even across the frame. 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==

Rudolf Kingslake and Paul Stevens filed for a patent in 1941 for the WA Ektanar, which featured a negative meniscus element facing the object, followed by a Tessar-derived four-element/three-group lens. However, the spacing between the first element and the following lens was relatively small and so the back focus was approximately equal to the focal length. Pierre Angénieux applied for a patent in 1950. In the original patent, he presented two lenses with an angle of view of 65°, approximately equal to the view of a f=35 mm lens on the 35mm format for still cameras; the first example had a maximum aperture of 2.5, while the second example had a maximum aperture of 2.2. The Angénieux corporation coined the name Retrofocus for its line of inverted telephoto wide-angles, and the name has become synonymous as a generic trademark for similar lens designs. File:Angénieux - Retrofocus (1950).svg|Angénieux Retrofocus R-1 (35 mm , 1950), from US Patent 2,649,022 File:Tronnier US2746351A (Skoparon, 1952).svg|Voigtländer Skoparon (35 mm , 1952) by Tronnier, from US Patent 2,746,351 File:Angénieux US2696758A (Retrofocus R-11, 1952).svg|Angénieux Retrofocus R-11 (28 mm , 1952), from US Patent 2,696,758 File:Lautenbacher US2983191A (Lithagon, 1953).svg|Enna Lithagon (35 mm , 1953) by Lautenbacher, from US Patent 2,983,191 File:Bertele US2772601A (Travegon, 1954).svg|Schacht Travegon (35 mm , 1954) by Bertele, from US Patent 2,772,601 File:Zöllner-Solisch US2793565A (Flektogon), 1955.svg|Zeiss Jena Flektogon by Zöllner & Solisch (1955), from US Patent 2,793,565 File:Klemt US2824495A (Curtagon, 1955).svg|Schneider Curtagon (28 mm , 1955) by Klemt, from US Patent 2,824,495 File:Schlegel DE1017382B (Eurygon, 1955).svg|Rodenstock Eurygon (35 mm , 1955) by Schlegel, from DE Patent 1,017,382 File:Determann US2927506A (Skoparet, 1956).svg|Voigtländer Skoparet (35 mm , 1956) by Determann, from US Patent 2,927,506 File:Solisch US2878724A (Westrogon, 1956).svg|ISCO Westrogon (24 mm , 1956) by Solisch, from US Patent 2,878,724 File:Eismann & Lange US3038380A (Distagon, 1958).svg|Carl Zeiss (Oberkochen) Distagon (35 mm , 1958) by Eismann & Lange, from US Patent 3,038,380 File:Angénieux FR1214945A (Retrofocus R-61, 1958).svg|Angénieux Retrofocus R-61 (24 mm , 1958), from French Patent 1,214,945 File:Dannberg & Dietzsch GB978797A (Flektogon, 1963).svg|Carl Zeiss (Jena) Flektogon (20 mm , 1963) by Dannberg & Dietzsch, from GB Patent 978797A Nikkor-H f=2.8cm lens (early 1960s); note large front element, characteristic of inverted telephoto designs The Angénieux Retrofocus lens line inspired other manufacturers to produce similar wide-angle lenses of this type for almost every 35mm SLR, helping to make it the definitive camera type of the late 20th century. Similar lenses with prominent object-facing meniscus lenses were patented a few years after the original Retrofocus patent. For example, Albrecht Tronnier used the Tessar-derived Skopar lens with a single negative meniscus for the Voigtländer Skoparon of 1952. In 1955, Harry Zöllner and Rudolf Solisch applied for a similar patent on an inverted telephoto lens design, branded Flektogon, for Carl Zeiss Jena, which uses a single negative meniscus element ahead of a Double-Gauss lens. These used multiple negative meniscus elements in the front group, a trend that would continue with the Zeiss (Jena) Flektogon 20 mm design of 1963, with three negative meniscus elements and an angle of view expanded to 94° from 62° (with the original Retrofocus R-1). Pincushion distortion is common with inverted telephoto designs because they are so highly asymmetric. Lee suggested using an air space in the negative group to control this distortion. By removing the constraint for rectilinear projection and deliberately introducing barrel distortion, the illumination of the field can be made more even; the resulting fisheye lenses can be considered a subset of the inverted telephoto lens design, with strong negative front elements. ==References==