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.
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 only conventional TLR cameras to feature truly interchangeable lenses. The Mamiya SLRs also employ bellows focusing, making extreme closeups possible.
Rollei Rolleiflex model TLRs have an additional feature for the "sports finder" that allows precise focusing. 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.
TLRs are different from single-lens reflex cameras (SLR) in several respects. First, unlike virtually all SLRs, TLRs provide a continuous image on the finder screen. The view does not black out during exposure. Additionally, models with leaf shutters rather than focal-plane shutters can synchronize with flash at higher speeds than can SLRs. However, because the photographer views through one lens but takes the photograph through another, parallax error makes the photograph different from the view on the screen. This difference is negligible when the subject is far away, but is critical for nearby subjects. For accuracy in tabletop photography, in which the subject might be within a foot (30 cm) of the camera, devices are available that move the camera upwards so that the taking lens goes to the exact position that the viewing lens occupied. This solves the parallax problem but it is still impossible to preview depth of field as one can with an SLR, as the TLR's viewing lens has no diaphragm.
A primary advantage of the TLR is its simplicity as compared to the more common single-lens reflex cameras. The SLR must employ some method of blocking light from reaching the film during focusing, either with a focal plane shutter (most common) or with the reflex mirror itself. Both methods add significant noise to the camera's operation. Most TLRs use a leaf shutter in the lens. The only mechanical noise during exposure is from the shutter leaves opening and closing. Most TLRs are also significantly lighter in weight than a medium format SLR.
Another advantage of the TLR design can be seen when long exposures are required. During exposure, an SLR's mirror must be retracted, blacking out the image in the viewfinder. A TLR's mirror is fixed and the taking lens remains open throughout the exposure, letting the photographer examine the image while the exposure is in progress. This can ease the creation of special lighting or transparency effects.
The TLR is especially useful for action portrait photography (e.g., martial arts portraits) as the action of the shutter can be very responsive to the photographer compared to the time required to move the mirror of an SLR. Owing to the availability of medium-format cameras and the ease of image composition, it is also preferred by many portrait studios for static poses.
The typical TLR is medium format, using 120 roll film with square 6×6 cm images. Presently, the Chinese Seagull Camera and the German Rollei are in production, but in the past, many manufacturers made them. 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 simple, sturdy construction of many TLRs means many have endured 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 TLR camera is the Swiss-made Tessina, using perforated 35mm film forming images of 14×21 mm.