Photographers used Single-Lens Reflex (SLR) cameras in the mid-nineteenth century. They effectively allowed photographers to see and focus by viewing the image in a 45 degree mirror. However, pulling the mirror up right before the exposure often jostled the camera. A faster, mechanical SLR camera solved this problem. Have you ever heard the “click” of a camera? That’s the sound of the mirror flipping up. SLR 35mm cameras became the most sought after cameras after World War II.
Before the invention of twin-lens reflex cameras, photographers had to insert the sensitized glass plate into the camera after focusing—potentially unfocusing the desired image plane as the film is put in place. A camera with two lens fixed this problem. By placing a mirror behind the second lens, photographers could see the image and focus with a viewfinder with out exposing the film behind the primary lens.
