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Why can a focal-plane shutter produce very short exposure times?

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Diagram showing the negative-positive photographic process: a scene is captured on film in a camera, developed into a negative, projected onto paper with an enlarger, and developed again to produce a positive print. — technology of photography Figure 1: Sequence of negative–positive process, from the photographing of the original scene to enlarged print (see text).

A focal-plane shutter achieves very short exposure times by utilizing a mechanism consisting of two blinds—or curtains—that move in succession across the image plane.12

Rather than opening the entire frame at once, these blinds form a traveling slit. The exposure duration is determined by the width of this slit: the narrower the slit, the shorter the exposure time. Because the actual travel time of the blinds remains relatively constant regardless of the chosen speed, the camera can achieve extremely brief exposures—sometimes as fast as 1/12,000 of a second—simply by narrowing the gap between the two blinds as they sweep across the sensor or film.12