includes a design for a lens with a 270° field of view. A 6.2 mm 5.6 SAP fisheye was later produced in limited numbers with an aspherical surface, encompassing a 230° field of view. 8 mm 2.8 (1970), the first circular fisheye with variable focus, automatic aperture, and reflex viewing (mirror lock-up no longer required). Meanwhile, other Japanese manufacturers were developing the so-called "full-frame" or diagonal fisheyes, which captured approximately a 180° field of view across the diagonal of the film frame. The first such diagonal fisheye was the Fish-eye Takumar 18 mm 11, released by Pentax (Asahi Optical) in 1962, followed by the slightly faster UW Rokkor-PG 18 mm 9.5 from Minolta in 1966. Both of these were reflex-viewing and fixed-focus. Both Pentax and Minolta followed up with faster lenses with variable focus in 1967 (Super Fish-eye-Takumar 17 mm 4) and 1969 (Rokkor-OK 16 mm 2.8), respectively. The 16 mm Rokkor was later adopted by Leica as the Fisheye-Elmarit-R (1974) and then converted to autofocus (1986) for the Alpha system. , the same basic optical design is still sold as the Sony SAL16F28. Design Unlike rectilinear lenses, fisheye lenses are not fully characterised by focal length and aperture alone. Angle of view, image diameter, projection type, and sensor coverage all vary independently of these. Image diameter and coverage In a circular fisheye lens, the image circle is inscribed in the film or sensor area; in a diagonal ("full-frame") fisheye lens, the image circle is circumscribed around the film or sensor area. This implies that using a fisheye lens for a different format than it was intended for is easy (as opposed to a rectilinear lens), and may change its characteristic. Further, different fisheye lenses map ("distort") images differently, and the manner of distortion is referred to as their mapping function. A common type for consumer use is equisolid angle. Although there are digital fisheye effects available both in-camera and as computer software, neither can extend the angle of view of the original images to the very large one of a true fisheye lens. Focal length The focal length is determined by the angular coverage, the specific mapping function used, and the required dimensions of the final image. Focal lengths for popular amateur camera sizes are computed as: Notes Circular fisheyes The first types of fisheye lenses developed were "circular" — lenses which took in a 180° hemisphere and projected