Nikkor-O 21mm (2.1cm) f/4 was designed by Mr. Zenji Wakimoto, a highly respectable Japanese lens designer known for the creation of Micro-Nikkor series lenses. The 21mm F/4 is a wide-angle lens with superb optical performance. Its optical design consist of eight elements in four groups with two inner three element groups encompasses the iris. Additional features include seven blade diaphragm, 0.9 meter close focus distance, 92° viewing angle and 52mm filter thread. The lens also requires the use of a mirror lockup and a separate optical viewfinder.