Although hardly the most technical of mechanisms the rotary blade shutter is important as it was produced in millions, most notably in countless cheap box cameras. It takes a number of forms but it is nothing more complicated than a rotating plate with a hole cut in it. A lever operated by the photographer tensions a spring which causes the disc to rotate sweeping the hole in front of the lens briefly. They mostly work on the "over centre" principle, operate in both directions and are limited to "timed" or "instant" shutter settings. There are many variations, but they mostly follow a similar principle and some designs survive into today's cheapest plastic disposable cameras.