In cameras, the term “reflex” refers to the use of a mirror inside the camera to reflect and change the direction of incoming light. The mirror ( or mirrors) redirects the light and creates an image on a ground glass screen, so the photographer can compose and focus the image. Simple twin-lens reflex cameras were developed as early as the 1870s, while the single-lens reflex camera’s success story, did not really begin until after World War II. With the development of digital single lens reflex cameras the popularity of reflex cameras continues into the 21st century and the digital age.
The twin-lens reflex camera is built with two lenses aligned vertically. The bottom lens takes the picture, while the overhead lens acts as a parallel focusing system. Behind the top lens is a fixed mirror that reflects the image at a right angle onto an optical screen at the top of the camera. To compose and focus the shot, the photographer looks straight down at this screen, which is parallel to the ground. The two lenses are connected in such a way that they both move as one unit; so any adjustments that are made affects both lenses equally. The era of the modern twin-lens reflex began with the development of the Rolleiflex in the 1920s and continued until the arrival of the SLR just after WWII.
The single-lens reflex camera allows the image maker to look directly through the lens that will take the picture. This is done with a movable, interior mirror that sits between the lens and film and redirects light to the top of the camera. At the height of the camera sits a complex “pentamirror” or prism-like device that focuses the image on a small screen, which can viewed from the exterior of the camera. After all adjustments are made, the photographer hit’s the release, which first swings the mirror out of the way before allowing light to pass through the shutter to the film. SLR cameras improved on the twin-lens reflex by showing the photographer the exact image that would be exposed onto the film.
Digital Single-Lens Reflex
In the Digital Single-Lens Reflex camera, 35mm film has been replaced with a light sensor and removable data storage, but the light mechanics are basically the same as they were in the older SLRs. There is still a movable mirror and a “pentamirror” on top, but new technology is on the way. This development comes in the form of a stationary translucent mirror that allows light to pass through only after the shutter release has been activated.