Neutral density filters (ND filters) are essential tools when it comes to shooting cityscapes at blue hour. Even without an ND filter, you could shoot for a few seconds of exposure (using a small aperture like f/13) when the light falls towards the end of dusk.
But those opaque filters let you take even longer exposure photos (minutes, not just seconds), and create beautiful effects such as light trails, silky smooth water, rushing clouds, etc., by slowing down the shutter speed by a... more