B+W polarization filters

... remove distorting reflections of light, increase color saturation, and in addition increase the contrast. The effect of the polarizing filter, "pol-filter" for short,  derives from its property of allowing the passage of only certain directions of vibration, but of absorbing other directions of vibration of light. In nature and landscape photography, the use of a polarizing filter conduces to a deep blue sky, brilliantly white clouds, and vivid colors in the case of flowers. It is also ideal when photographing reflecting surfaces of water. The numerous possibilities of application apply both to black-and-white- and color photographs, and in digital photography as well. 

Working with polarizing filters, however, really leads to results only if additional filters are used. A light blue conversion filter (KB 1.5), together with a pol-filter, can make cloud formations bright and sunny, and give the whole picture a cheerful character. By contrast, a weak red conversion filter (KR 1.5) can produce a dramatic, stormy mood in a color photograph. The B+W Redhancer intensifies the orange, red, and brown colors of the subject. For that reason, it is the ideal filter for, e.g., for landscape photography in autumn.

The pol-filter can achieve a striking effect if the light on the subject itself is already partially polarized (vibrating in a privileged direction). The light reflected from the shiny surfaces of transparent materials, like glass or water, but also from painted surfaces and the surfaces of plastics, is polarized. The oldest generally known application of polarizing filters is the elimination of reflections coming from glass. To be sure, the degree of elimination is dependent on the camera angle with respect to the surface of the glass.

Picture 1.  Photograph without polarizing filter ...	Picture 2.  ... taken with polarizing filter

A special phenomenon is the polarization of the light from the sky. The degree of polarization is dependent on the angle toward the sun. If the sun is directly overhead, then the sky beyond the horizon is strongly polarized. If the sun is close to the horizon, the effect of the polarizing filter can be expected to be strongest when the light comes from the side. The polarization of the from the sky is depends very greatly on the state of the atmosphere. It achieves its highest value in very clear air, i.e., in areas far from cities and industry. 

Haze and darkening of the atmosphere work to the disadvantage of the polarization of the light of the sky. If the sun is behind a completely pale blue to white sky, the polarization effect is very weak. If the sky is yellowish white, then there is no polarization of the sky. In this case the application a polarizing filter for intensifying the color of the sky is not advisable.

In connection with the light of the sky, one can, if one uses a wide-angle lens , sometimes achieve different levels of intensity in the sky; it then shows a rising and falling of the degree of polarization in the angle toward the sun. If one would like to use a polarizing filter in order to eliminate reflections from the surface of the water, it must be kept in mind that here, too, the degree of elimination of reflections depends on the camera angle. With rough seas or bubbling mountain springs, with constantly changing camera angles, no successful application is possible. With still bodies of water, however, the use of the pol-filter makes sense, not only in order to photograph objects under water, but also to strengthen or weaken reflections,  depending upon the setting of the filter. 

Metal surfaces have only a slight polarizing effect on light; consequently, a significant elimination of reflections is not to be achieved. Plastics often exhibit rainbow-colored, bizarrely formed stripes and colored patterns. The cause is the inner tension of these plastics, which can be intensified through pressure. 

B+W pol-filters are made in various types and dimensions, both in the form of linear as well as circular pol-filters. Basically, both consist of a linearly polarizing film. With a circular polarizing filter, an additional retardation film is added, which places the linearly polarized light into rotation, thereby preventing erroneous measurements in optical measurement systems ; hence, for cameras with internal devices for light meters (TTL) and/or  autofocus, only circular polarizing filters can be used! The same   applies to digital cameras and camcorders.

By turning the pol-filter in front of the lens, it can completely block certain vibration levels of light, or at least reduce them visibly. The optimal setting can be seen in the view-finder of a single-lens reflex camera, or on the monitor of a digital camera.

Physical background

Light can be imagined as light particles (photons) moving linearly in wave form - in most cases it vibrates completely randomly. From this unpolarized light, a privileged direction can be suppressed by means of a polarizing filter (pol-filter). The light then vibrates only on one level, as Picture 4 makes clear.

Picture 3. Structure of a pol-filter: a polarization film is cemented between two filter glasses.		Picture 4. Mode of operation: light of only one plane of vibration is allowed to pass through.

A pol-filter can be imagined as an arrangement of many lattices, and the polarization layer only works when, by stretching or pulling in one direction, the molecules of the plastic (e.g., PVA, polyvinyl alcohol)or crystals are brought into a uniform ordered position. If the lattice is in a vertical position, only those particles can pass through which are vibrating vertically; those vibrating horizontally are held back because they are too extended in this direction. Therefore, only those photons can pass through which happen to be in a favorable position. 

Thus, a polarizing filter differs from a color filter in that the latter absorbs light waves of certain lengths (colors) instead of waves moving in different directions.

Picture 5.  A circular polarization filter in a 5 mm super-flat SLIM screw mounting of the Käsemann type.