Optical aberration refers to the imperfections in the imaging properties of optical systems, such as lenses and mirrors, that prevent them from focusing all incoming light to a single point. These aberrations result in distortions or blurriness in the images produced by these optical devices. There are several types of optical aberrations, each affecting image quality in different ways.
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An optical aberration is a departure from ideal image formation: light rays from one object point may miss their intended common image point, or image positions may depart from the intended mapping. Spherical aberration, coma, and astigmatism describe different ways of spreading a point's light rays; optical distortion can change image shape without spreading each point. In physical optics, these departures can be described by wavefront error relative to a reference wavefront. Diffraction also gives a finite point spread function in a perfect optical instrument, so an extended image alone does not establish an optical aberration.