Neutral Density Filters
When it is desirable to maintain a particular lens opening for sharpness or depth-of-field purposes, or simply to obtain proper exposure when confronted with too much light intensity, use a Neutral Density (ND) filter. This will absorb light evenly throughout the visible spectrum, effectively altering exposure without requiring a change in lens opening and without introducing a color shift.
A Neutral Density filter reduces the amount of light passing through the camera lens without changing the color of the scene. It is especially useful in bright light conditions to help prevent overexposure. The neutral density filter also allows proper exposure at a wider lens opening for reduce depth-of-field to highlight a key subject by making the foreground and/or background out of focus.
The Neutral Density filter also allows shutter speeds to produce blurred motion effects.
Available in densities .3, .6 and .9 - These correspond to 1, 2 and 3 stops light reduction respectively and are sometimes represented as x2, x4 and x8.
Also available are Graduated Neutral Density filters.