Heliopan 48mm Slim Mount, Wide Angle Circular Polarizer Filter.
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Polarizing filters, are color neutral and increase picture taking possibilities with black and white or color film. In color photography, unlike black and white photography, there is no possibility to change contrast or color saturation with colored filters. To do so would also change the colors in the photograph. Natural light spreads in a wave form in all direction. Sunlight is polarized and has a high color purity. When passing through the atmosphere a part of it is scattered, which results in a blush gray haze over all colors. This effect can be reduced or eliminated by using polarizing filters. If, in addition, light falls at an angle of approximately 35° onto a reflecting surface, e g. water, glass, plastic, color surfaces, with the exception of metallic surfaces, the light becomes reflected and polarized. If one uses a properly orientated polarizing filter, the reflections are absorbed and the true colors can reach the film. Water surfaces become transparent and clouds are more saturated.
The light loss of a polarizing filter varies between 1 and 1.5 F stops which, depending on camera type, can be manually or automatically corrected. HELIOPAN polarizing filters consist of a neutral color foil cemented between two plane parallel glass discs. The foil is constructed so that light rays can only pass in one plane. The polarizing filter is mounted in a calibrated rotating ring so the desired position and effect can be easily set. With an SLR one sees the result in the finder, with viewfinder type cameras one holds the polarizer in front of the eye and turns it to see the desired effect. Then with the assistance of the calibrated rim, which is on every HELIOPAN polarizing filter, it is easy to set the filter to the same position on the lens. If no effect is visible, the available light conditions can not be polarized. In this case its use is not advisable.
In principle there are two different kinds of polarizer's, linear and circular. The circular is required with camera systems where the built-in light measuring system uses light that has passed through the mirror via a beam splitter. In this case the light is already partially polarized, on all AF-cameras. Using linear polarizing filters cause in correct focusing measurements. The circular polarizer has a second component, a 1/4 wave plate in its construction. The polarized light is set in rotation by this plate and can therefore pass without interruption through the measuring system and give proper measurements.
Circular polarizer's are also recommended with the auto focus lenses in video cameras. Over heating of the polarizing filter should be avoided.