The Singh-Ray 67mm Vari-N-Duo Variable Neutral Density and Warming Circular Polarizer Filter is a filter that combines a solid variable neutral density element with a warming circular polarizer element. The solid variable neutral density element provides an exposure reduction of 2.6 to 8 stops. Its 0.8 to 2.4 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. The degree of density is easily controlled by rotating the rear filter ring, helping you to predetermine the additional exposure length required. By slowing your exposure time or increasing your aperture, you are able to control depth of field and convey movement more easily.
Above this filter's solid variable neutral density element is an independently-rotating warming circular polarizer element that can be applied to create a warm cast over an entire image while helping to 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 element 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.
The Singh-Ray 67mm Vari-N-Duo Variable Neutral Density and Warming Circular Polarizer Filter's warming effect can be obtained under daylight when this filter is used on a film camera system with daylight film installed, or on a digital camera system with its daylight white balance setting selected. The coloration of this filter's warming circular polarizer element corresponds to an ability to reduce 3400K color temperatures to 3200K, which reflects a mired shift value of +18 that can be applied to other starting color temperatures as well. To maintain image clarity, optical glass set within aluminum-alloy rings that are durable while remaining lightweight is used for the construction of this filter.