TLR stands for Twin Lens Reflex. The camera uses two lenses of equal focal length, one for viewing and focusing and the other for taking the photograph; reflex refers to the mirror used behind the viewing lens that makes focusing possible. The great commercial successes among TLR designs have been the 6×6 cm designs that flourished from the 1930s to the 1950s, but TLR designs long predated these.
Twin Lens Reflex (TLR) cameras are “two-eyed” cameras such as the Rolleiflex or Rolleicord. They normally consist of two lenses of equal focal length and equal “speed”. They are mounted one above the other in the front of the camera, and their focusing is synchronized so that they are always focused at the same distance. The difference is that the upper lens projects the incoming image via mirror up to the reflex finder’s ground glass focusing screen while the lower one (the “taking” lens) projects the image into the camera’s dark chamber and onto the film plane. The taking lens can be stopped down while the finder lens is always at maximum aperture.
The scene viewed by the top lens is reflected by a mirror onto the ground glass screen. This is usually viewed from above, in a waist-level viewing hood. When viewed like this, the image on the screen is laterally inverted (i.e. left-right), which can take some time to get used to. This phenomenon is neither unique to nor an inevitable characteristic of TLR cameras; a waist-level finder on an SLR camera will give the same left-right reversal. Prism finders, which both correct this reversal and allow eye-level focusing, were available for some Rolleiflex and Mamiya TLR cameras.
The taking lens exposes the film, usually by an in-lens leaf shutter. The small distance between the centre of the viewing lens and that of the taking lens leads to a difference in the scene each captures; this phenomenon is known as parallax error; if the subject of the photographs permit, this can be corrected by lifting the camera until the taking lens is as high as the viewing lens was when the image was composed.