For longer exposures without changing the aperture the variable ndx filter allows the freedom to change the shutter speed according to the effect desired for example without adjusting the aperture the same 160 scene can be taken using the variable ndx filter with shutter speeds from 120 to 6 seconds with just a turn of the outer filter ring the problem of diffraction is eliminated by being able to work with a slow shutter speed for wider apertures with the variable ndx filter the shutt