Single Lens Reflex. A type of camera that has one (switchable) lens that a) projects the image to the film and b) projects the image to the viewfinder.

The finder part is done with a mirror and prism: Upon press-motion of the Button That Causes The Kodak Moment To Be Captured Forever and Ever, the mirror that reflects the image to the prism (and thus to the viewfinder) moves away, permitting the image to be projected through the shutter to the film.

SLR is kind of cool because the picture that's visible from viewfinder is precisely - lens focus, iris, all that jazz - the same that will go to the film (most cheap cameras have separate finder, you know, makes getting perfect pictures in all environments much harder!). My sister noted the noise it makes, though - she said I can always be found easily if I take photos, because of the loud *CLANK!* every time I take one. (My camera is from late 1960s, but I know the modern SLRs aren't much quieter. =)

'SLR' was also the British Army's pet name for their version of the FN FAL. Officially designated the L1A1, it was universally known by soldiers as the 'SLR', which stood for 'Self Loading Rifle'. Until its introduction, the British Army had been armed with the Lee-Enfield SMLE, a bolt-action rifle (which meant that the soldier had to pull back a bolt in order to load the rifle after each shot). The SLR, on the other hand, 'loaded itself', hence 'self loading rifle'.

In camera terms, a 'reflex' camera is one in which the user focusses whilst looking through the lens - reflexively, as it were, without having to interpret a distance scale. The earliest reflex cameras were TLRs, 'Twin Lens Reflex' cameras, which had two lenses coupled with a gear; one for taking the picture, and one for focussing (as such, a TLR is essentially a clever rangefinder camera).

SLRs date back to the turn of the century and it would appear that there is no 'first ever' SLR; the first modern pentaprism-based 35mm SLR was the 1949 Contax S, examples of which fetch $300 or so on eBay.

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