Astigmatism

• What is astigmatism?

  An anomaly of refraction causing two focal lines
• How does astigmatism affect the image of a point object?

  It creates two focal lines instead of one
• What are the refractive surfaces involved in astigmatism?

  The cornea and lens
• What describes the orientation and power of astigmatism?

  The steepest and flattest curves
• What are the principal meridians in astigmatism?

  The steepest and flattest curves
• What is the "Interval of Sturm" in astigmatism?

  The dioptric distance between the line foci
• Where does the circle of least confusion lie?

  Midway between the two principal foci
• What is the point of least blur in astigmatism?

  The circle of least confusion
• Can spherical lenses correct astigmatism?

  No, they cannot correct astigmatism
• What type of lenses are required to correct astigmatism?

  Cylindrical lenses
• What are the categories for classifying astigmatism?

  1. Position of focal lines relative to retina
  2. Orientation of focal lines
  3. Contributing components of the eye
  4. Magnitude
  5. Separation of meridians of maximum and minimum power
• What is simple astigmatism?

  One foci on the retina, one off
• What characterizes compound astigmatism?

  Both foci are in front or behind retina
• What defines mixed astigmatism?

  One foci in front, one behind retina
• How can you identify simple astigmatism from a prescription?

  If the sphere is plano or becomes plano
• What indicates simple myopic astigmatism?

  Sphere is plano, cylinder is minus
• What indicates simple hypermetropic astigmatism?

  Sphere is plano, cylinder is plus
• How is compound myopic astigmatism identified?

  Sphere is minus, regardless of cylinder sign
• How is compound hypermetropic astigmatism identified?

  Sphere is plus, regardless of cylinder sign
• What indicates mixed astigmatism?

  Sign of sphere changes when transposed
• What characterizes 'With the Rule' astigmatism?

  Steepest meridian is vertical (70-110°)
• What characterizes 'Against the Rule' astigmatism?

  Steepest meridian is horizontal (160-20°)
• What is astigmatism?

  An anomaly of refraction where the image of a point object is not a single point, but rather two focal lines that are generally mutually perpendicular
• Why are the refractive surfaces (cornea and lens) not spherical in astigmatism?

  They are described by the orientation and power of the steepest and flattest curves, known as the principal meridians
• What is the "Interval of Sturm" in astigmatism?

  The dioptric distance between the line foci
• Where is the circle of least confusion located in astigmatism?

  It lies midway between the two principal foci
• How can spherical lenses be used to correct astigmatism?

  Spherical lenses can move the circle of least confusion to the retina but they cannot correct the astigmatism
• What type of lenses are required to correct astigmatism?

  Cylindrical lenses
• What are the different types of astigmatism based on the position of the focal lines relative to the retina?

  • Simple astigmatism: One focus is on the retina (plano power) and one is in front or behind the retina
  • Compound astigmatism: Both foci are in front or behind the retina
  • Mixed astigmatism: One focus is in front of the retina and one is behind
• What is the defining characteristic of simple myopic astigmatism?

  The sphere is plano and the cylinder is minus
• What is the defining characteristic of simple hypermetropic astigmatism?

  The sphere is plano and the cylinder is plus
• What is the defining characteristic of compound myopic astigmatism?

  The sphere is minus regardless of the sign of the cylinder
• What is the defining characteristic of compound hypermetropic astigmatism?

  The sphere is plus regardless of the sign of the cylinder
• What is the defining characteristic of mixed astigmatism?

  The sign of the sphere changes when the prescription is transposed
• How can you determine the type of astigmatism from a prescription?

  • Simple astigmatism: Sphere is plano or becomes plano when transposed
  • Compound myopic: Sphere is minus regardless of cylinder sign
  • Compound hypermetropic: Sphere is plus regardless of cylinder sign
  • Mixed: Sphere changes sign when transposed
• What are the three main orientations of astigmatism?

  With-the-rule, against-the-rule, and oblique
• What is the defining characteristic of with-the-rule astigmatism?

  The steepest meridian is vertical (between 70-110°)
• What is the defining characteristic of against-the-rule astigmatism?

  The steepest meridian is horizontal (between 160-20°)
• What is the defining characteristic of oblique astigmatism?

  The steepest meridian is neither vertical nor horizontal (21-69° or 111-159°)
• How are the different types of astigmatism corrected?

  • With-the-rule: Corrected with -ve cyl with horizontal axis or +ve cyl with vertical axis
  • Against-the-rule: Corrected with -ve cyl with vertical axis or +ve cyl with horizontal axis
  • Oblique: Corrected with -ve cyl or +ve cyl in an oblique orientation