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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.〔http://www.dpreview.com/news/0403/04030502rolleiminidigi.asp〕 == History == 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〔http://www.tlr-cameras.com/Japanese/slides/Koni-Omegaflex.html〕 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.〔http://www.tlr-cameras.com/history.htm〕 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.〔"Complete Collector's Guide to the Rollei TLR, Ian Parker, Hove Photo Books, Jersey, 1993〕 TLRs are still manufactured in Germany by DHW Fototechnik, the successor of Franke & Heidecke in three versions. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Twin-lens reflex camera」の詳細全文を読む スポンサード リンク
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