Coma, shown below in a computer-generated mesh, can be thought of as a variation in magnification with aperture; the
image distortion increases with the distance of the marginal rays from the optical axis.
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A wavefront with coma
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In spherical lenses, different parts of the lens surface exhibit different degrees of magnification. This gives rise to an
aberration known as coma. As shown in the figure below, each concentric zone of a lens forms a ring-shaped image called a
comatic circle. This causes blurring in the image plane (surface) of off-axis object points. An off-axis object point is
not a sharp image point, but it appears as a characteristic comet-like flare.
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Imaging an off-axis point source by a lens with positive transverse coma
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Even if spherical aberration is corrected and the lens brings all rays to a
sharp focus on axis, a lens may still exhibit coma off axis, as seen below.
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Positive transverse coma
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As with spherical aberration, correction can be achieved by using multiple surfaces. Alternatively, a sharper image may
be produced by judiciously placing an aperture, or stop, in an optical system to eliminate the more marginal rays.
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