SLR (or Single Lens Reflex) cameras started to become popular in the 1970’s and they differed from non-SLR cameras in one important aspect. Up until this time all cameras with viewfinders had two optical light paths: One path through the objective lens straight to the film and another path through another objective lens at the front, ending at the viewfinder, which was either positioned above (TLR or twin-lens reflex, TLR camera) or to the side (rangefinder) of the camera.
These two objective lenses (with exactly the same vocal length) were installed in such a way that the viewing lens and the film lens would intersect at a particular point in front of the camera, but any object closer would be a problem. Due to the effect of parallax framing errors occur in close-up shots; in other words, what the photographer sees in the viewfinder will not be exactly the same as what he eventually gets in the picture in terms of the boundaries of the picture.
With the introduction of SLR cameras only one objective lens would be used for both the shot and for viewing (thus “SL”- “single lens”- in “SLR”): From now on the viewfinder used exactly the same optical path as the one ending up at the film or electronic imaging sensor (CCD or CMOS).
Light coming in at the lens at the front is reflected (thus the “R” – “reflex” – in “SLR”) by a mirror into a prism (which corrects the image), finally ending up at the eyes of the photographer in the viewfinder, which is situated at the top of the camera. Unlike all pre-SLR cameras, the viewer is now looking at exactly the same image as what will eventually be imprinted on the film or imaging sensor.
The mirror actually shields the light coming in at the lens from the film or imaging sensor. The moment the shutter, shielding all light from the sensitive film or sensor at the end of the optical path, is released to let light through, the mirror is moved out of the way. This has never been done before, since prior to SLR cameras two optical paths have been used instead of one.
Nowadays the term “SLR” is virtually always associated with digital cameras, although it didn’t start out that way.
One can also get a better understanding of what SLR cameras are by comparing them with modern compact (inexpensive) digital point-and-shoot non-SLR cameras, which became available on the market since the early 1990’s. Virtually all of these compact digital cameras have live LCD preview screens, allowing the photographer to see what the image sensor is capturing, and have actually overshadowed SLR’s as far as popularity goes.
However, SLR cameras have maintained their position as the preferred choice of dedicated amateur and professional photographers. The main reason for this is the fact that SLR cameras are so-called “system cameras”. This means that they have interchangeable parts, allowing customization.
At the very least you can add interchangeable lenses, but also a number of other things, like a separate electronic external flash, remote shutter release, extensive supplementary equipment for macro photography, adapters for third-party lenses, interchangeable viewfinders, extra-capacity battery packs, as well as fully-external packs with cable interface, depth-of-field preview, GPS receivers and Bluetooth or Wifi networking modules.
In addition to all these benefits SLR digital cameras also have a better image sensor (CMOS, CCD in the case of compact non-SLR’s), far less shutter lag, better pixel resolution and contrast ratio. Furthermore, the color gamut of an LCD preview screen is inferior to the clarity and shadow detail of a direct-viewed optical SLR viewfinder.
With a compact non-SLR digital camera your choices are limited as far as composing a shot; with an SLR digital camera there’s virtually no restriction; controlling the aperture and depth-of-field, to name but two features, are excellent examples. But by this time I’m sure you’re getting the picture!