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 (the lens that takes the picture), while the other is used for the waist-level viewfinder system. 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.
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 TLRs exist, since their heyday ended long prior to the digital era. The main exception is the collector-oriented Rollei Mini-Digi, introduced as a rather expensive "toy" in 2004.
Double-lens cameras seem first to have been developed around 1870, when someone realised that having a second lens alongside the taking lens meant that one could focus without having to keep swapping a ground glass screen for the plate afterwards, making the time delay in actually taking the shot rather less. This sort of approach was still used as late as the 1960s, as the monstrous Koni-Omegaflex testifies.
The TLR as such was an evolution using a reflex mirror to allow viewing from above, allowing the camera to be held much more steadily if handheld. The same principle of course applied to the SLR, but early SLRs caused delays and inconvenience through the need to move the mirror out of the focal plane to allow light to the plate behind it. When this process was automated, the movement of the mirror could cause shake in the camera and blur the shot. The London Stereoscopic Co's "Carlton" model is claimed to have been the first off-the-shelf TLR, dating from 1885.
The major step forward to mass marketing of the TLR came with the Rolleiflex in 1929.
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 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. The Mamiya TLRs also employ bellows focusing, making extreme closeups possible.
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