retardation plate

Definition: Optical element used in filters: light incident on the filter emerges as circularly polarized light. * Consists of a thin layer of dielectric material which changes the phase of the electric vector of a light wave: the emerging beam recombines electric and magnetic vectors to form a polarized beam whose character depends on the degree of retardation induced. Retardation accelerates use of auto-focusing The phase-difference systems in auto-focus cameras depend on the analysis of split beams of light: splitting a beam of light always causes some plane polarization. So if you try to use a normal polarizing filter on an AF SLR, you completely screw up the AF system: more like than not, the polarized beam entering the AF system will be crossed with respect to the polarization inherent to the system, so drastically reducing brightness -- exactly what happens when you cross the polarizer to darken skies. Fortunately, you can avoid the problem by first circularly polarizing light ... and a retardation plate (more likely layer, really) is the way to do this.

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