Astigmatism optical aberration:
Astigmatism aberration occurs, “If focal points in the horizontal plane and vertical plane of an image does not coincide”. (i.e Horizontal plane has different focus and vertical plane has different focus).
System with astigmatism aberration:
Both the tangential plane and sagittal plane are perpendicular to each other.
Tangential focus is perpendicular to tangential plane which has proper focus in tangential plane. To avoid confusion look at the below diagram
Sagittal focus is perpendicular to sagittal plane which has proper focus in sagittal plane.
In case of system with no aberration, the tangential and sagittal focus of an images lie on the same surface (A function of the index of refraction and the surface curvatures of the lens elements).
Properties of astigmatism aberration:
- The best focus of an image can be found at circle of least confusion (Note: Circle of least confusion is also called as Disk of least confusion).
- Astigmatism aberration affects image sharpness.
James C Wyant, Katherine Creath, “Applied Optics and Optical Engineering,” Academic Press Inc, Vol. XI, pp. 03-39, ISBN 0-12-408611-X, 1992.