FP sync refers to a type of flash synchronization once used with focal plane flashbulbs. It ensures that the flash fires at the correct moment to properly expose the film while the shutter curtains move across the focal plane.
FP sync was particularly important for high-speed photography with focal plane shutters, allowing photographers to capture sharp images without uneven exposure. While largely obsolete today due to modern electronic flash systems, understanding FP sync is useful for those working with vintage cameras or studying the history of photographic lighting techniques.