A polarizing filter is an optical filter accessory that appears neutral grey in colour and is mounted in a rotating ring that allows it to be turned through 360 degrees while attached to the front of a camera lens, enabling the photographer to continuously vary the angle of the filter's polarizing axis relative to the polarized light in the scene and thereby control the degree to which polarized light is blocked or transmitted. By rotating the filter to the angle at which its transmission axis is perpendicular to the plane of polarization of unwanted reflected light, the photographer can selectively reduce or completely eliminate specular reflections from non-metallic surfaces, deepen the saturation of blue skies, improve overall colour saturation and contrast, and reduce the veiling effect of atmospheric haze in distant landscapes.
The polarizing filter works by exploiting the selective transmission properties of its polarizing layer, which passes light vibrating in its transmission direction while absorbing light vibrating in the perpendicular direction. When the filter is rotated so that its transmission axis is aligned with the plane of polarization of unwanted reflected light, that reflected light is blocked and the reflection is eliminated or substantially reduced. When the filter is rotated 90 degrees from this position, the transmission axis is perpendicular to the polarization of the reflected light, which can now pass through the filter freely, and the reflection is restored to its full intensity. Between these two extremes, the filter provides a continuously variable degree of reflection control, allowing the photographer to choose any intermediate level of reflection reduction to suit the creative and technical requirements of the shot.
The most immediately striking and widely used application of the polarizing filter in outdoor photography is the darkening and saturation of blue sky areas. Skylight scattered by the atmosphere is partially polarized, with the maximum degree of polarization occurring in regions of sky approximately 90 degrees from the direction of the sun. When the camera is pointed at right angles to the sun and the polarizing filter is rotated to its maximum effect position, the partially polarized skylight is substantially blocked by the filter, causing the blue sky to appear significantly darker and more saturated in the image while white clouds remain unaffected, producing the deep blue sky and brilliant white cloud contrast that is one of the most recognisable and sought after effects in landscape and travel photography. This sky darkening effect is only achievable optically with a polarizing filter - no amount of post processing adjustment can fully replicate the selective blocking of polarized skylight that the filter produces at the point of capture.
The control of reflections from water, glass, and other non-metallic surfaces is another of the polarizing filter's most practically valuable capabilities. By eliminating the surface reflection from a body of water, the filter allows the photographer to see and record detail beneath the surface - fish, rocks, submerged vegetation, and the colour of the water itself - that would otherwise be completely obscured by the reflected sky and surroundings. Similarly, eliminating reflections from shop windows, glass display cases, and glazed artworks allows the photographer to capture the subject behind the glass without the distracting overlay of reflected surroundings that would otherwise compromise the image. It should be noted that polarizing filters are not effective at reducing reflections from metallic surfaces, which reflect light without polarizing it and are therefore unaffected by the filter's selective transmission properties.
Polarizing filters are available in two optical types that differ in the arrangement of their internal components and their compatibility with different camera systems. Linear polarizing filters contain a single polarizing layer that produces linearly polarized transmitted light, and while optically effective, they can interfere with the beam splitting prisms used in the through the lens metering and autofocus systems of modern SLR and mirrorless cameras, causing inaccurate exposure readings and unreliable autofocus performance. Circular polarizing filters, which are the standard recommendation for use with all modern SLR and mirrorless cameras, add a quarter wave plate element after the linear polarizing layer that converts the linearly polarized transmitted light to circularly polarized light before it enters the camera body, preserving the full polarizing effect on external reflections while preventing interference with the camera's internal optical systems.
The polarizing filter reduces the overall light transmission to the film or sensor by approximately one and a half to two stops, regardless of the rotation angle, as the filter always blocks at least half of the randomly polarized light in any scene even when set to its minimum effect position. This light loss must be compensated by adjusting the exposure, and cameras with through the lens metering systems will automatically account for this reduction when metering through the filter. The filter factor varies slightly with the specific filter and scene conditions, and the variable density of the polarizing effect as the filter is rotated means that the light transmission changes slightly with rotation, requiring the exposure to be assessed with the filter already set to the desired rotation angle.