An optical aberration is a distortion in the image formed by an optical system compared to the original. They can arise for a number of reasons having to due with the limitations of optical components such as lenses and mirrors.

Astigmatism occurs in lenses because a lens has different focal lengths for rays of different orientations, resulting in a distortion of the image. In particular, rays of light from horizontal and vertical lines in a plane on the object are not focused to the same plane on the edges of the image.

Chromatic aberration occurs in lenses because lenses bring different colors of light to a focus at different points.

Coma occurs because off-axis rays no not quite converge at the focal plane. Coma is positive when off-axis rays focus furthest from the axis, and negative when they are closest.

Distortion is caused because the transverse magnification may be a function of the off-axis image distance. Distortion is classified as positive (so-called pincushion distortion), or negative (so-called barrel distortion).

Field curvature (a.k.a. Petzval field curvature) results because the focal plane is actually not planar, but spherical.

Spherical aberration occurs in a spherical lens or mirror because these do not focus parallel rays to a point, but instead along a line. Therefore, off-axis rays are brought to a focus closer to the lens or mirror than are on-axis rays.

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