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,’ 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. 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 realized 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. 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 major step forward to mass marketing of the TLR came with the Rolleiflex in 1929. The Rolleiflex was widely imitated and copied and most mass-market TLRs owe much to its design. 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.
The Daily Omnivore 
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