The 35 mm film-based Nikon F, 1959, the world's first single-lens reflex camera
A twin-lens reflex camera (TLR) is a type of camera with two objective lenses of the same focal length. One of the lenses is the photographic objective or "taking lens" (the lens that takes the picture), while the other is used for the viewfinder system, which is usually viewed from above at waist level. In addition to the objective, the viewfinder consists of a 45-degree mirror (the reason for the word reflex in the name), a matte focusing screen at the top of the camera, and a pop-up hood surrounding it. The two objectives are connected, so that the focus shown on the focusing screen will be exactly the same as on the film. However, many inexpensive TLRs are fixed-focus models. Most TLRs use leaf shutters with shutter speeds up to 1/500th sec with a B setting.The vast majority of TLRs take 120 film and expose 12 pictures in 6×6cm format. Some models did take 127 film,and expose 12 pictures in 4×4 format, and there are a few models using other formats. No general-purpose digital TLR cameras exist.