Camera Filters
Neutral Density filters
In brighter locations you may want to reduce the amount of light passing through your lens. If you want a shallower depth of field you may not want to close down the aperture completely or get that choppy sharp look of a fast shutter speed. A neutral density filter reduces light by 1 or more stops without changing the color.
0.3 ND filter* 1 f-stop
0.6 ND filter 2 f-stops
0.9 ND filter 3 f-stops
1.2 ND filter 4 f-stops
Polarizing Filters
These filters can reduce reflections and glare, deepen the color of blue skies, penetrate haze, and saturate colors.
(--Cybercollege.com)