With early cameras, the glass film plates were so large that cameras and stands needed to be carried around on horses. Bellows shifted the lens back and forth, the image was viewed on a fine ground glass, the photographer would duck under a dark cloth to see the image on the ground glass clearly, especially outdoors. Then the ground glass would be replaced with film, and the shot taken.
Film improved, became smaller; so did cameras. But to become hand-held, they needed quicker, more accurate focusing. Enter the rangefinder, a highly accurate distance measurement device (already being used in cartography and weaponry). It used two lenses in two windows, at a distance that roughly mimicked our eyes. Via a mirror and prism assembly, the two images overlapped within a single viewfinder; rotating a dial shifted mirror and prism, and when the images overlaid perfectly, the lensing could be set. Later, this was built into the camera: Rotating the focus ring on the lens let the rangefinder overlay images to achieve perfect focus.
With Kodak’s Autographic Special in 1916, the world was introduced to the concept of accurate focusing in a compact, affordable body. A decade later, Leica created history with their rangefinder designs, combining extremely good optics with small, rugged builds. They became the camera of choice for professionals. (Classic Leica designs are copied even today for digital cameras!) Contax, Canon, Konica, Olympus, Nikon, Minolta and Rollei followed with excellent cameras, marking a golden age for rangefinders from the ’40s to the ’60s. The legendary Henri Cartier-Bresson was known to always use a Leica rangefinder; Pablo Picasso had a FED-2; Queen Elizabeth II took her Rollei 35 everywhere.