The F-21 is a famous Soviet-made photo camera for concealed use; a spy camera in simple words. This model was in production at KMZ plant from 1951 to mid-1990's. This camera was designed and made with its purpose in mind - it is very small, light, can shoot the entire roll of film from one fill wind of its spring-loaded motor. Picture quality was, of course, very important as well, so the F-21 features a very sharp, high-resolution lens and a reasonably large negative size. The frame is 18x24 mm, taken on a 21 mm film. So technically, using the 35 mm terms, the F-21 is a half-frame camera.
There were many attachments for the F-21, all intended to conceal it in different ways. There are belt buckles, coat buttons, purse and bag adapters. There are ordinary 35 mm cameras and umbrellas that could accommodate the small F-21. Some accessories, like the button, were made in small quantities, some - in single units. The standard lens is OF-28T, 28/2.8, coated. There are 4 shutter speeds - B, 1/10, 1/30, 1/100. Produced in 1951-1995. (source: www.fedka.com).