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 "pseudo" TLRs are fixed-focus models. Most TLRs use leaf shutters with shutter speeds up to 1/500th sec with a B setting.
For practical purposes, all TLRs are film cameras, most often using 120 film, although there are many examples which used other formats. No general-purpose digital TLR cameras exist, since the heyday of TLR cameras ended long before the era of digital cameras. The main exception is the collector-oriented Rollei Mini-Digi, introduced as a rather expensive "toy" in 2004.
Double-lens cameras were first developed around 1870, due to the realization that having a second lens alongside the taking lens would mean that one could focus without having to keep swapping the ground glass screen for the plate, reducing the time required for taking a picture. This sort of approach was still used as late as the 1960s, as the monstrous Koni-Omegaflex testifies.
The TLR camera was thus an evolution. Using a reflex mirror to allow viewing from above also enabled the camera to be held much more steadily than if it were to be held in the hand. The same principle of course applied to SLR cameras, but early SLR cameras caused delays and inconvenience through the need to move the mirror out of the focal plane to allow light to pass to the plate behind it. When this process was automated, the movement of the mirror could cause shake in the camera and blur the image. The London Stereoscopic Co's "Carlton" model is claimed to have been the first off-the-shelf TLR camera, dating from 1885.
The major step forward to mass marketing of the TLR came with the Rolleicord and then Rolleiflex in 1929, developed by Franke & Heidecke in Germany. The Rolleiflex was widely imitated and copied and most mass-market TLR cameras owe much to its design. It is said that Reinhold Heidecke had the inspiration for the Rollei TLRs whilst undertaking photography of enemy lines from the German trenches in 1916, when a periscopic approach to focusing and taking photos radically reduced the risk to the photographer from sniper fire.
Higher-end TLRs may have a pop-up magnifying glass to assist the user in focusing the camera. In addition, many have a "sports finder" consisting of a square hole punched in the back of the pop-up hood, and a knock-out in the front. Photographers can sight through these instead of using the matte screen. This is especially useful in tracking moving subjects such as animals or race cars, since the image on the matte screen is reversed left-to-right. It is nearly impossible to accurately judge composition with such an arrangement, however.
Mamiya's C-Series, introduced in the 1960s, the C-3, C-2, C-33, C-22 and the Mamiya C330 and Mamiya C220 along with their predecessor the Mamiyaflex, are the main conventional TLR cameras to feature truly interchangeable lenses. "Bayonet-mount" TLRs, notably Rolleis & Yashicas, had both wide-angle and tele supplementary front add-ons, with Rollei's Zeiss Mutars being expensive but fairly sharp. Rollei also made separate TLRs having fixed wide-angle or tele lenses: the Tele Rollei and the Rollei Wide, in relatively limited quantities; higher sharpness, more convenient (faster than changing lenses) if one could carry multiple cameras around one's neck, but much more costly than using 1 camera with supplements. The Mamiya TLRs also employ bellows focusing, making extreme closeups possible.
Many TLRs used front and back cut-outs in the hinged top hood to provide a quick-action finder for sports and action photography. Late model Rollei Rolleiflex TLRs introduced the widely-copied additional feature of a second-mirror "sports finder". When the hinged front hood knock-out is moved to the sports finder position a secondary mirror swings down over the view screen to reflect the image to a secondary magnifier on the back of the hood, just below the direct view cutout. This permits precise focusing while using the sports finder feature. The magnified central image is reversed both top-to-bottom and left-to-right. This feature made Rolleis the leading choice for press photographers during the 1940s to 1960s.
The typical TLR is medium format, using 120 roll film with square 6×6 cm images. Presently, the Chinese Seagull Camera is still in production along with Lomography's Lubitel, but in the past, many manufacturers made them. DHW-Fototechnik GmbH, continues to make the Rolleiflex TLR, as well (http://www.dhw-fototechnik.de/en/rolleiflex-tlr.html). The Ciro-flex produced by Ciro Cameras Inc. rose dramatically in popularity due in large part to the inability to obtain the German Rollei TLRs during World War II. The Ciro-flex was widely accessible, inexpensive, and produced high quality images. Models with the Mamiya, Minolta and Yashica brands are common on the used-camera market, and many other companies made TLRs that are now classics. The Mamiya C series TLRs had interchangeable lenses, allowing focal lengths from 55mm (wide angle) to 250mm (telephoto) to be used. The bellows focusing of these models also allowed extreme closeups to be taken, something difficult or impossible with most TLRs. The simple, sturdy construction of many TLRs means they have tended to endure the years well. Many low-end cameras used cheap shutters however, and the slow speeds on these often stick or are inaccurate.
There were smaller TLR models, using 127 roll film with square 4×4 cm images, most famous the "Baby" Rolleiflex and the Yashica 44. The TLR design was also popular in the 1950s for inexpensive fixed focus cameras such as the Kodak Duaflex and Argus 75. Though most used medium format film, a few 35mm TLRs were made, the Contaflex TLR being the most elaborate, with interchangeable lenses and removable backs.
The smallest general-use TLR camera is the Swiss-made Tessina, using perforated 35mm film forming images of 14×21 mm. It has been argued that the "business end" of the Olympus Gastro Camera is technically the smallest actual TLR device.
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