SLR cameras are small wonders of modern technology. This neat video shows a shutter in action, so you can see exactly how the mechanism works.
The general concept behind SLR shutters is pretty simple: cover the sensor/film, uncover it for the proper exposure time, then cover it up again. In practice, coordinating the two shutter curtains to follow each other at precise intervals consistently over the course of hundreds of thousands of exposures is an impressive feat.
So, why two shutter curtains? There are two reasons: first, for faster shutter speeds, the exposure is so short that by the time the first curtain clears the sensor, it's already too late, so a second curtain follows closely behind it, essentially forming a slit that travels across the sensor. In fact, the fastest shutter speed in which the entire sensor is simultaneously exposed for some period of time is known as the sync speed of the camera. Second, having two curtains ensures even exposure across the entire sensor; if there was only one curtain that opened, then reversed direction, the area of the shutter near the bottom of the curtain's path of travel would receive less exposure than the top.
Check out the video above to see it in action!
[via ISO 1200]