Astigmatism
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Astigmatism, pictured below in a computer-generated mesh, occurs when the
tangential and sagittal foci do not coincide and the system appears to have two
different focal lengths. |
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An astigmatic wavefront |
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When an off-axis object is focused by a spherical lens, the natural asymmetry leads to
astigmatism. As shown in the figure below, the plane containing both optical axis and object point is called the tangential plane. Rays that lie in this plane are called tangential rays. Rays not in this plane are referred to as skew rays. The chief, or principal, ray goes from the object point through the center of the aperture of the lens system. The plane perpendicular to the tangential plane that contains the principal ray is called the sagittal or radial plane. The figure illustrates that tangential rays from the object come to a focus closer to the lens than do rays in the sagittal plane. When the image is evaluated at the tangential conjugate, we see a line in the sagittal direction. A line in the tangential direction is formed at the sagittal conjugate. Between these conjugates, the image is either an elliptical or a circular blur. Astigmatism is defined as the separation of these conjugates. The amount of astigmatism in a lens depends on lens shape only when there is an aperture in the system that is not in contact with the lens itself. (In all optical systems there is an aperture or stop, although in many cases it is simply the clear aperture of the lens element itself.) Astigmatism strongly depends on the conjugate ratio. |
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Astigmatism represented by sectional views |
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| Optics Guide Copyright 2002 Melles Griot Inc. |