well. The camera was so popular that it bred over 500 imitations, more than half of them from Japan. The factory grew rapidly; by 1956 – the year the millionth camera was sold – the workforce numbered 1600, and by 1957 the workforce had grown to 2000 employees. Development of the Rolleiflex Hans Hass, the underwater diving pioneer, approached Franke & Heidecke to see if they could make him a special housing suitable for underwater photography. As a result, Rollei developed the Rolleimarin, an ingenious underwater camera housing rated for depths up to 100 m. It was made from two cast metal parts. The top part contained a glass prism that was attached to the camera's focusing screen. There were also knobs on top of the housing for adjusting the exposure and aperture settings. On the bottom, left-hand side of the housing was the focusing knob, and on the right-hand side, the winding handle and a frame counter. There was also a filter turret. A special flash bulb could be attached for flash photography, in which case a battery pack had to be installed inside the housing. Of course, a viewfinder frame could also be screwed onto the housing. Although there were many imitators, none could match the quality of the original Rolleiflex – that is, until the Mamiya C-Series from Japan appeared in 1956. Mamiya initially offered three sets of interchangeable double lenses for it: normal, telephoto and wide-angle. Later on, additional double lenses with focal lengths of 55 mm and 250 mm were introduced; one even had a dimmable viewfinder lens, which enabled depth of field adjustment through the viewfinder. By comparison, the Rolleiflex only had a single, fixed normal lens, although the Rollei Magnar tele-converter lens with 4x magnification was available as an accessory. This could be attached to the front of the primary lens while a mask (which did not magnify the image) was placed over the focusing screen. In addition, Zeiss offered two double lens converters which attached to the viewfinder filter bayonet and the primary lens bayonet. The 5-element Mutar tele-converter magnified 1.5×, weighed 327 g and could render an image accurately in the viewfinder up to a distance of 4m from the object. The 4-element Mutar wide-angle converter magnified 0.7×, weighed 437 g and could render an object accurately from a distance of 1 meter. In all cases, for the best image quality,