Description of Astigmatism
An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different focus. Without correction, the optical system where magnification is only to one dimension produces an astigmatism, where the vertical and horizontal lines will be in sharp focus at two, slightly different distances, thus compromising the sharpness.
Astigmatism can be much improved in prism design by a fixed additional optical element (P-100 series), and corrected precisely with the cylinderical lens design of continuously movable lens element (C-100 series).
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