Great for use when the light in bright and you want (for example) to create a slow shutter speed shot (water falls , moving traffic etc) with motion blur.

Coming in a range of darkness (grey) they need not be expensive but are a welcome addition to the camera bag. On www.7dayshop.com a 55mm ND4 can be as low as £3:99. This website is very good, the stock comes from Jersey so is VAT Free and arrives in 3 to 5 days.

For more info on ND Filters this link might help:

ND Filters - click here