Originally Posted by Nicole McGrade
A rangefinder camera is one that uses a set of prisms to triangulate the distance to the subject by making two images at a given offset come together in a viewfinder. This is mechanically connected to the lens, and as the lens is focused, the two images are superimposed. Some older cameras have separate rangefinders and viewfinders, but newer ones tend to combine the two. The viewfinder has a set of lines to show what's in the frame, and the interchangeable lens rangefinder cameras have multiple frames available for different focal length lenses, or can use auxiliary flast-shoe mounted finders. The split image focus common in some single lens reflex cameras is a form of rangefinding device.
Rangefinders have no viewing pentaprism or mirror to move from behind the lens on taking an exposure, so you can see the subject during the exposure, and there is often less shutter lag in a rangefinder design. They also don't require special lens designs at short focal lengths to make room for a mirror box, so those designs can be more straightforward. They are _generally_ lighter, smaller, and faster handling than an SLR of equivalent format. They do come in formats from smaller than 35mm through at least 4x5. Look up the Leica, Nikon, Canon, and Minolta 35mm rangefinders, lots of Fuji 670 and 690 models, Mamiya 7, many Graflex 4x5's were available with rangefinders, Plaubel-Makina 67, and The Fuji/Hasselblad Xpan. Literally hundreds of models of folding medium format camera models from about 1900 up through the 50's were rangefinders.
Rangefinder benefits are faster, more accurate focusing on mid to wide focal lengths, fast handling, lower sound levels, light weight, and small size. Drawbacks are somewhat less accurate framing, poor handling/framing with lenses longer than short telephotos, and most are fairly basic cameras if you prefer the automatic stuff. There are a few autofocus/autoexposure rangefinders, although some would not call an autofocus camera a true rangefinder. Most rangefinder shooters prefer them because once they know the camera, it stays out of the way, or at least makes it easier to get what you want quickly and discretely. There has been a rangefinder "rennaisance" in the last 5 - 10 years, especially in 35mm, where new, lower priced cameras and lenses have rejuvenated the market.