A single lens reflex camera (SLR camera) uses only one lens for both viewing and picture-taking. The image is reflected onto a viewing screen by a moveable mirror inside the camera. Just before the camera's shutter opens, the mirror flips out of the way, allowing the film to receive light. The image you see through the lens is the image you get in the photograph.
SLR cameras have been around since the early1900s. However, it was in the 1970s when they advanced technologically and became the main type of camera used by serious amateur photographers and professionals. SLRs have camera bodies and can use various interchangeable lenses from wide angle to telephoto. They can also use various accessories such as tripods, remote shutter releases, motor film advances and flashes. Most new models come with a small built in flash, but are capable of using larger detachable flashes. SLR cameras are very versatile as they offer automatic, semi-automatic or manual control over the shutter speed, aperture and focus.
Digital SLRs are also available and are quite popular.