astigmatism: one construct, many systems

In ophthalmology, astigmatism is a refractive error in which the cornea or lens is not rotationally symmetric but more steeply curved in one meridian than in the perpendicular one, so light in the two planes focuses at different distances and a point object forms two separate line foci rather than a single sharp point, blurring vision at every distance. Thomas Young described the astigmatism of his own eye in his 1801 Bakerian Lecture On the mechanism of the eye. It is corrected with a cylindrical lens, and the referent is a property of the living eye's optics, specified by a cylinder power and an axis.[1]

Unit

The dioptric difference between the eye's two principal meridians, plus its axis

Recovery means

Not applicable

Measured by

Measured by subjective refraction, keratometry, and corneal topography, giving cylinder power and axis

In lens design, astigmatism is a monochromatic off-axis (Seidel) aberration in which rays in the tangential and sagittal planes come to focus at different axial distances, so an off-axis point is imaged as two perpendicular line foci separated along the axis, with a blurred disk of least confusion between them. It is treated in Born and Wolf's Principles of optics. This is the same two-line-foci defect as in the astigmatic eye (that it is genuinely one construct across the two systems is our reading), but here it arises from oblique incidence on an otherwise symmetric lens rather than from a toric surface.[2]

Unit

The axial separation of the tangential and sagittal foci (the astigmatic difference)

Recovery means

Not applicable

Measured by

Quantified by the Seidel astigmatism coefficient and the tangential–sagittal focal separation across the field