Definition: Defect or imperfection of optical image caused by inabilities of lens system to form perfect image. Collective name for deviations of image from that predicted by geometrical (Gaussian or paraxial) optics. * See spherical aberration, coma, astigmatism, distortion, field curvature, chromatic aberration, diffraction.
Rationalising aberrations
A perfect lens is one displaying no aberration. This means (1) it reproduces a point in the subject as a point (2) it projects a flat plane facing square on to the lens as a flat plane (3) it reproduces a straight line anywhere in the image field as a straight line (4) it reproduces accurately all the colours in the object, without bias or deficiency. (We wish!)

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