0.9 Neutral Density Standard reduces exposure by 3 stop without affecting colour balance.
ND standards can be classed as essential in modern photography. These filters absorb light evenly across the visible spectrum, therefore reducing the light entering the lens without affecting the image colour.
Traditionally used on film for extending shutter speeds in bright conditions, they are now becoming essential to digital photographers for reducing the sensitivity of sensor chips on cameras which have relatively high minimum ISO settings.
ND Standards extend exposure times allowing for movement in an image, be it people, water, foliage or clouds. If used with care the effects can be impressive.