Enables exposure to be lengthened; With rotating front lens; For reducing light if the brightness is too high; Enables exposure to be lengthened between 1 & 8 aperture stops One neutral-density filter for more fields of application; Allows the photographer to use large apertures while taking pictures outside backround gets hazy; In case of exposure times longer than 130 seconds water "flows" (if they are shorter it freezes to ice) Note for Consumers To avoid vignetting this filter has a superstructure of the front lens ie the diameter of the front lens is larger than that of the rear lens Please note that a lens cap with 52 mm is thus required