This 138mm Schneider Mounted Neutral Density Circular Polarizer Filter is a filter that combines a solid neutral density effect and a circular polarizer effect within a single glass surface. The solid neutral density effect provides an exposure reduction of 3 stops. Its 0.9 density creates a darkening of the entire image, allowing you to photograph with a wider aperture or slower shutter speed than would normally be required. By slowing your exposure time or increasing your aperture, you are able to control depth of field and convey movement more easily.
Along with this filter's solid neutral density effect is a circular polarizer effect that is simultaneously applied to help reduce reflections and glare by filtering out light that has become polarized due to reflection from a non-metallic surface. The light from the sun naturally becomes partially polarized due to reflecting off electrons in air molecules, causing the light to scatter into what appears as haze. A polarizing filter arranges, and filters, this directionally polarized light perpendicularly to the reflected light, allowing for the absorption of much of this light. This results in a noticeable reduction of glare and reflections from non-metallic surfaces and an increase in the saturation of skies and foliage.
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.
This filter is constructed from water white glass for increased optical clarity as well as color fidelity. It is 138mm in diameter and mounted for use in third-party or custom filter holders. No front or rear threads are included on this filter's ring, or mount.