A circular polarizer only differs from a linear polarizer in that it contains a quarter wave plane in addition to the linear polarizing filter. The addition of this quarter wave plane is to circularly polarize the linearly polarized light for the use of auto focus and exposure functions. Due to how the light is broken down and filtered with a linear polarizer, auto focus and exposure accuracy become skewed during use. When the light becomes circularly polarized, it permits the use of these functions.
1.8 neutral density filter darkens the image by 6 stops, allowing you to photograph with a longer shutter speed or wider aperture than normally required.
