Twin-lens reflex cameras (TLR) have two lenses of identical focal length. One lens takes the photos while the other is used for the viewfinder system. It is called a reflex camera because the viewfinder uses a mirror, a matte focusing screen and a hood surrounding it. Many TLRs have pop-up magnifying glasses to help you focus. Some have a "sports finder" which is a square hole punched in the back of the hood and a knock-out in the front. You can view through these instead of the matte screen. This helps you follow moving objects since the image on the screen is reversed.
Twin lens reflex cameras differ from single-lens reflex cameras as most TLRs show a continuous image on screen, which doesn't black out during exposure. Versions with leaf shutters can synchronize with flash at higher speeds than SLRs. Because you view through one lens, but take the photo through another, the photograph differs from the view on screen. This is important for close subjects and devices that move the photo taking lens to the position the viewing lens was in are available. TLRs are generally medium format cameras and use 120 roll film with 6 x 6 centimeter images. A Seagull made twin lens reflex camera camera can be bought for about $150.
