The front of a Kinaflex twin-lens reflex camera

A twin-lens reflex camera (TLR) is a type of camera characterized by having two objective lenses of the same focal length. One of the lenses is the actual photographic objective (the lens that takes the picture), while the other is used for the viewfinder system. In addition to the objective, the viewfinder consists of a mirror (the reason for the word reflex in the name), a matte focussing screen at the top of the camera, and a pop-up hood surrounding the focussing screen. The two objectives are connected, so that the focus shown on the focussing screen will be exactly the same as on the film. Many TLRs have a pop-up magnifying glass to assist the user in focussing 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, since the image on the matte screen is reversed left-to-right.

TLRs are different from single-lens reflex cameras (SLR) in several respects. First, unlike most 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.

The typical TLR is medium format, using 120 roll film with square 6 x 6 cm images. Presently, the Chinese Seagull 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.