As it moves, the first curtain passes across the film frame, allowing light to fall on the film. When the first curtain has completed its travel, the frame is fully opened. When exposure time ends, the second curtain is released to begin its travel and close off light to the film. Basic construction is comprised of two opaque curtains are mounted on rollers. Normally, the first curtain is positioned so it covers up the surface of the film and does not allow light to reach it.
Film frame is covered by first curtain.
First curtain begins to travel across frame, opening frame to image from lens. To make an exposure, the first curtain travels from one roller to the other. The first curtain moves across the film frame and stops at a location that leaves the frame open to receive light from the lens. First curtain travel completed. The entire film frame is exposed to light.