it as a circle within the film frame. By design, circular fisheye lenses thus cover a smaller image circle than rectilinear lenses designed for the same sensor size. The corners of a circular fisheye image will be completely black. This blackness is different from the gradual vignetting of rectilinear lenses and sets on abruptly. Some circular fisheyes were available in orthographic projection models for scientific applications. These have a 180° vertical, horizontal and diagonal angle of view. For APS and m43 cameras, several lenses have emerged that retain a 180° field of view on a crop body. The first of these was the Sigma 4.5mm. Sunex also makes a 5.6mm fisheye lens that captures a circular 185° field of view on a 1.5x Nikon and 1.6x Canon DSLR cameras. Nikon produced a 6 mm circular fisheye lens for 35 mm film that was initially designed for an expedition to Antarctica. It featured a 220° field of view, designed to capture the entire sky and surrounding ground when pointed straight up. This lens is no longer manufactured, and is used nowadays to produce interactive virtual-reality images such as QuickTime VR and IPIX. Because of its very wide field of view, it is very large — weighing , having a diameter of , a length of and an angle of view of 220 degrees. It dwarfs a regular 35 mm SLR camera and has its own tripod mounting point, a feature normally seen in large long-focus or telephoto lenses to reduce strain on the lens mount. The lens is extremely rare. More recently, the Japanese manufacturer Entaniya offers several fisheye lenses with angles of view up to 250° on 35 mm full frame, and up to 280° on smaller sensors (see list below). In 2018, Venus Optics introduced a 210° fisheye lens for the Micro Four Thirds system. The 8 mm and 7.5 mm circular fisheye lenses made by Nikon have proven useful for scientific purposes because of their equidistant (equiangular) projection, in which distance along the radius of the circular image is proportional to the zenith angle. Diagonal fisheyes (a.k.a. full-frame or rectangular) As fisheye lenses gained popularity in general photography, camera companies began manufacturing fisheye lenses with an enlarged image circle to cover the entire rectangular film frame. They are called diagonal, or sometimes "rectangular" or "full-frame", fisheyes. (This was well before digital photography, so the use of the term