The Twin Lens Reflex
There have been no digital versions, but twin lens reflex (TLR) film cameras date back to the earliest days of photography and became a very popular design for around four decades from the 1920s. The legendary Rolleiflex TLRs, first introduced in 1928, was arguably the most successful model range, surviving until the end of the 20th century and widely used by both amateurs and professionals. Most used rollfilm although there were a handful of 35mm models.
As the name suggests, the TLR has two lenses arranged inline one above the other. The top lens is used for viewfinding, via a reflex mirror, and the bottom lens takes the picture. Because these two roles are separated, the reflex mirror can be fixed which made these cameras less complex to manufacture and quieter in their operation. Typically, a TLR has a waistlevel viewfinder (which you look down into), but some professional-level models could be fitted with eyelevel finders. Most TLRs had fixed lenses, but Mamiya offered interchangeable lens models (which were obviously fitted with twin lens sets).
Today only a few diehard enthusiasts still use rollfilm TLRs, but they remain popular as collectible classic cameras, especially the numerous models of Rolleiflex.