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Astigmatism and Cylindrical Lenses

Optics & Refraction

Teaching images

Exam pearl

Analogy: Imagine a perfectly round football - light entering it focuses at a single point. Now imagine a rugby ball: it has TWO different curvatures along two perpendicular axes. Light hitting a rugby-ball-shaped cornea focuses at TWO different focal lines, not one point. The space between these two focal lines is the Sturm's Conoid - the optical signature of astigmatism. Correcting astigmatism means identifying WHICH axis is steepest and flattening it optically with a cylinder lens.

Astigmatism is a refractive error in which the refracting power of the eye is not uniform in all meridians, resulting in the inability to form a single point focus on the retina.
The image of a point source of light is projected as two focal lines (Focal Lines of Sturm) separated by the Interval of Sturm.

A cylinder lens has power in ONE meridian only and is plano (zero power) in the perpendicular meridian.
The axis of a cylinder is the meridian of NO power.
In minus-cylinder form, the axis is parallel to the flatter corneal meridian; in plus-cylinder form, it is parallel to the steeper meridian.

Term	Definition	Clinical Relevance
Principal Meridians	• The two perpendicular meridians of maximum and minimum power	• Basis of astigmatic correction
Axis of Cylinder	• Meridian of zero cylinder power • 180° system, TABO notation	• Depends on cylinder sign
Power Meridian	• Meridian of maximum cylinder power • Perpendicular to axis	• Corrects the steep corneal meridian
Circle of Least Confusion	• Cross-section midway between two focal lines • Least blurred image	• Targeted in spherical equivalent prescription
Interval of Sturm	• Distance between the two focal lines along optical axis	• Greater interval = higher cylinder power
Spherical Equivalent (SE)	• Sphere + ½ cylinder • Focuses circle of least confusion on retina	• Used selectively for soft contact-lens prescribing when manifest astigmatism is very low or absent • Not a general substitute for full astigmatic correction in children at risk of amblyopia
Jackson Cross Cylinder (JCC)	• ±0.25 or ±0.50 D crossed cylinder • Refines axis and power	• Flip test: equal blur = endpoint
Keratometry (K-readings)	• Measurement of anterior corneal curvature/power in two principal meridians • The difference estimates anterior corneal astigmatism	• Directly assesses anterior corneal meridional power

Term	Definition	Clinical Relevance
Principal Meridians	• The two perpendicular meridians of maximum and minimum power	• Basis of astigmatic correction
Axis of Cylinder	• Meridian of zero cylinder power • 180° system, TABO notation	• Depends on cylinder sign
Power Meridian	• Meridian of maximum cylinder power • Perpendicular to axis	• Corrects the steep corneal meridian
Circle of Least Confusion	• Cross-section midway between two focal lines • Least blurred image	• Targeted in spherical equivalent prescription
Interval of Sturm	• Distance between the two focal lines along optical axis	• Greater interval = higher cylinder power
Spherical Equivalent (SE)	• Sphere + ½ cylinder • Focuses circle of least confusion on retina	• Used selectively for soft contact-lens prescribing when manifest astigmatism is very low or absent • Not a general substitute for full astigmatic correction in children at risk of amblyopia
Jackson Cross Cylinder (JCC)	• ±0.25 or ±0.50 D crossed cylinder • Refines axis and power	• Flip test: equal blur = endpoint
Keratometry (K-readings)	• Measurement of anterior corneal curvature/power in two principal meridians • The difference estimates anterior corneal astigmatism	• Directly assesses anterior corneal meridional power

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