Which shutter mechanism is known for faster exposure speeds, moving across the film?

The focal plane shutter is specifically designed to allow for faster exposure speeds by moving across the film plane. This mechanism consists of two curtains that move horizontally (or vertically in some cameras) across the sensor or film. When the shutter is activated, the first curtain opens to allow light in, and then the second curtain follows to block the light, effectively controlling the duration of exposure. This design allows for fast shutter speeds, which is particularly useful in scenarios requiring quick capture to avoid motion blur or achieve a specific effect.

In contrast, the intervalometer refers to a device used to automate the taking of a series of photographs at set intervals, which does not pertain to the speed of exposure itself, but rather to timing. The aerial camera shutter specifically refers to shutters used in aerial photography, which may have different mechanisms and are not universally faster than focal plane shutters. Lastly, the term diapositive relates to a type of photographic slide, not to a shutter mechanism at all. Thus, the focal plane shutter stands out as the correct answer due to its capability for rapid exposure through its unique moving design.