The hottest thing in the digital camera store is well the digital Slr, which is great 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 complicated 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 straight through the lens, focused, composed and then inserted a singular film plate behind the optics to make an image. While the whole process was crude by contemporary standards, the photographer enjoyed great control, since he looked directly straight through the actual imaging lens to build the shot.
While this was fine for still life, portraits and landscapes, this process did not lend itself to operation photography. These early cameras could only narrative a singular 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 assorted 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 someone else problem. The roll of film had to pass intimately behind the camera's optics, which meant that the photographer could no longer look straight through the camera lens to build and focus.
Rangefinder cameras appear to keep things in focus
The lower-end, consumer roll-film cameras ordinarily used an cheap "fixed-focus" lens, so a straightforward viewfinder was sufficient. great capability optics, however, require the lens to be focused, and since the photographer could not look straight through the lens with a roll-film camera, this was a major problem. One of the first solutions to this qoute 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 -- ordinarily with very good results.
Twin Lens Reflex cameras offer someone else solution
While the rangefinder type cameras worked well, the camera business is all the time evolving. A second formula of allowing the photographer to focus and build 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 straight 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 well look straight through the lens. This sometimes made exact mixture difficult.
Slrs take cameras someone else step forward
In their quest to allow users to see straight through the actual "taking" lens, camera makers turned to the periscope -- a straightforward gadget using two mirrors located at opposite angles to bend the light path. Periscopes are easy to understand -- any kid can build one from a concentrate of mirrors and some scrap wood.
In a camera, the lower mirror is located at a 45 degree angle directly behind the lens. Light remarkable 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 straight through the viewfinder on a Slr, you look straight 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 someone else wrinkle. They had to move that mirror. Just long adequate to make an exposure, since when the mirror moved, the photographer could no longer see whatever straight 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 improbable feat, when you reconsider 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 build their images by looking straight 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.
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