EN: Aberration.


Failing in the ability of a lens to produce a true image. There are many forms ofaberration and the lens designer can often correct some only by allowing others toremain. Generally, the more expensive the lens, the less its aberrations (More attentionto optical quality). While no single lens is called a perfect lens . The ideal lense would reproducea subject in a faithful, clearly defined image on film. Aberrations, which can bedivided into six basic faults, affect the Ideal performance in an optical system.a) Spherical aberration. Basically, a beam of light passing through a lensparallel to the optical axis converges to form 3 focused image on the film. Sphericalaberration is the term for an optical fault caused by the spherical form of a lensethat produces different focus points along the axis for central and marginal rays.b) Curvature of field.This optical defect causes points on an object plane perpendicular to the lens axisto focus on a curved surface rather than a plane.c) Astigmatism. Rays of light from a single point of an object which is noton the axis of a lense fail to meet in a single focus thus causing the image of apoint to be drawn out into two sharp lines, one radial to the optical axis and anotherperpendicular to this line, in two different planes near the curvature of field.d) Coma. This opticaldefect causes the image of an off-axis point of light to appear as a comet-shapedblur of light. Coma, as well as curvature of field and astigmatism, degenerate theimage forming ability of the lense at the rims of the picture.e) Distortion. Even if the first four aberrations were totally eliminated,images could result that still have a distorted appearance. For an example, an rectanglemay appear as a barrel or pin cushion-shaped object.f) Chromatic aberration. This aberration is caused by light rays of differentwavelengths coming to focus at different distances from the lense. Blue will focusat the shortest distance and red at the greatest distance. Since the natural raysof light are a mixture of colors, each aberration will give a different value correspondingto each color thus producing blurred images.
Mir glossary