B+W Neutral Density filters, or grey filters, are used if too much light restricts creative possibilities. They reduce the light in accordance with their density; they therefore allow longer exposure times or more widely opened apertures.
These filters are excellent for enabling movement to be captured in an image – by allowing longer exposure times. For example water can appear soft and smoky, rather than frozen in time.
Grey filters can create ingenious effects in architecture or urban photography too, with long exposures times of several hours, people walking through the shot are blurred or can even ‘disappear’ completely.