Topogon design, also designed by Richter for Zeiss in 1933. Testing on a captured lens after the war showed the Pleon provided an equidistant projection to cover a field of approximately 130°, and negatives were printed using a special rectifying enlarger to eliminate distortion. The Pleon had a focal length of approximately 72.5 mm with a maximum aperture of 8 and used a plano-concave front element in diameter; the image on the negative was approximately in diameter. 35 mm development At approximately the same time that Schulz was developing the Weitwinkelobjektiv at AEG, at Zeiss was developing the Sphaerogon, which was also designed to encompass a 180° field of view. Unlike the Weitwinkelobjektiv, Merté's Sphaerogon was not limited to medium format cameras; prototype versions of the Sphaerogon were constructed for the Contax I miniature format camera. The first prototype Sphaerogon lenses constructed had a maximum aperture of 8, but later examples were computed half a stop faster, to 6.8. Several prototype examples of Sphaerogon lenses were recovered as part of the Zeiss Lens Collection seized by the Army Signal Corps as war reparations in 1945; the collection, which the Zeiss firm had retained as a record of their designs, was later documented by Merté, the former head of optical computation for CZJ, working under Signal Corps officer Edward Kaprelian. The Nikon Fisheye Camera was discontinued in September 1961, and Nikon subsequently introduced the first regular production fisheye lens for miniature cameras in 1962, the Fish-eye-Nikkor 8 mm 8, which required the reflex mirror on its Nikon F and Nikkormat cameras to be locked up prior to mounting the lens. Prior to the early 1960s, fisheye lenses were used primarily by professional and scientific photographers, but the advent of the fisheye for the 35 mm format increased its popular use. The Nikkor 8 mm 8 has a field of view of 180° and uses 9 elements in 5 groups; it has a fixed focus and built-in filters intended for black-and-white photography. Research indicates that fewer than 1,400 lenses were built. Nikon subsequently released several more milestone circular fisheye lenses in Nikon F mount through the 1960s and 70s: 10 mm 5.6 OP (1968), the first fisheye to feature orthographic projection, which was also the first lens to feature an aspherical element 6 mm 5.6 (1969), the first fisheye to feature a 220° field of view; interestingly, the patent accompanying this lens