The hottest thing in the digital camera market is undoubtedly the digital SLR, which is better known as a dSLR. While dSLRs are flying off dealer shelves, many new users are confused about the terminology. Most people know that SLR stands for "single lens reflex." Since nearly all SLRs accept interchangeable lenses, it would appear they should be known as multiple lens reflex (MLR) cameras.
If you want to understand how the SLR received it's name, you have to dip into the history of the camera. Early cameras were similar to the view cameras used today. The photographer looked through the lens, focused, composed and then inserted a single film plate behind the optics to make an image. While the entire process was crude by modern standards, the photographer enjoyed great control, since he looked directly through the actual imaging lens to compose the shot.
While this was fine for still life, portraits and landscapes, this process did not lend itself to action photography. These early cameras could only record a single image at a time. Which is why you have never seen a motor-driven view camera.
Realizing the need to offer sequences of exposures, camera makers begin to experiment with various roll-film designs. With a roll of film in the camera, the photographer could fire off numerous images without reloading. Although this improved throughput dramatically, it caused another problem. The roll of film had to pass closely behind the camera's optics, which meant that the photographer could no longer look through the camera lens to compose and focus.
Rangefinder cameras appear to keep things in focus
The lower-end, consumer roll-film cameras generally used an inexpensive "fixed-focus" lens, so a simple viewfinder was sufficient. Better quality optics, however, require the lens to be focused, and since the photographer could not look through the lens with a roll-film camera, this was a major problem. One of the first solutions to this problem was the Rangefinder -- a type of camera that offered a distance measuring scale in the viewfinder. By determining the range from the viewfinder, the photographer could then adjust the focus to match -- usually with very good results.
Twin Lens Reflex cameras offer another solution
While the rangefinder type cameras worked well, the camera industry is always evolving. A second method of allowing the photographer to focus and compose appeared in the "Twin-Lens Reflex" cameras. These cameras used two identical lenses, arranged one on top of the other in the manner of an over-and-under shotgun. The film winds past the lower lens, while the photographer can focus through the upper lens. The twin-lens cameras were fairly bulky, so designers added a mirror and ground glass to the top of the camera, hence the term "reflex.
Now the user could hold the camera at waist level and look down at the ground glass which previewed the image via the mirror behind the upper lens. As the user adjusted the focus on the upper lens, a gear mechanism moved the lower "taking lens" to match.
While both rangefinders and twin-lens reflex cameras offered a credible way to focus and preview a shot, neither allowed the photographer to actually look through the lens. This sometimes made exact composition difficult.
SLRs take cameras another step forward
In their quest to allow users to see through the actual "taking" lens, camera makers turned to the periscope -- a simple device using two mirrors placed at opposite angles to bend the light path. Periscopes are easy to understand -- any kid can construct one from a couple of mirrors and some scrap wood.
In a camera, the lower mirror is placed at a 45 degree angle directly behind the lens. Light striking the mirror is projected upwards to a ground glass. While a second mirror would show the image on the ground glass to the user, it would not appear right, because mirrors tend to reverse things. So camera designers added a prism arrangement that corrects the reversed image. When you peer through the viewfinder on a SLR, you look through a prism, which displays the image on a ground glass, which displays the projected image from the mirror located behind the lens.
There is just one problem. If you have been paying attention, you have no-doubt realized that the lower mirror blocks the light path to the film (or digital sensor as the case may be.) Now the photographer can look though the lens, but the image cannot be projected on to the filmplane.
So the camera designers had to add another wrinkle. They had to move that mirror. Just long enough to make an exposure, since when the mirror moved, the photographer could no longer see anything through the lens. So they designed the "instant-return" mirror. At the instant of exposure, the mirror flies upward, the shutter fires and the mirror snaps back down. It is a incredible feat, when you consider that instant return mirrors have to flip up and back in a heartbeat, over and over for the life of the camera.
Once the instant return mirror was perfected, photographers could once again design their images by looking through the lens. Unlike the twin lens reflex, this new breed of camera needed only one lens to focus and shoot with. So they became known as... you guessed it.... Single-Lens Reflex cameras.
