Spherical vs Toric Cornea
A healthy cornea curves like a basketball; an astigmatic one curves like a football.
- Spherical: Normal shape (equal curvature all meridians)
- Toric: Astigmatic shape (football-like, two curvatures)
- ~1 in 3: Prevalence (people have notable astigmatism)
- Genetic: Common cause (shape of cornea or lens)
Why shape matters
Placeholder: curvature uniformity determines focus quality.
Two Meridians With Different Curvature
The steepest and flattest meridians are usually perpendicular to each other.
- Higher D: Steep meridian power (bends light more)
- Lower D: Flat meridian power (bends light less)
- 90°: Typical angle (apart between meridians)
- Keratometry: Measured by (or corneal topography)
Meridian mapping
Placeholder: topography maps power across the corneal surface.
Light Splitting Into Two Focal Lines
Instead of one focal point, astigmatism produces two separated focal lines.
- Conoid of Sturm: Structure name (classic optics model)
- Least confusion: Between the lines (blur circle midpoint)
- Perpendicular: Line orientation (to each meridian)
- Cyl power: Depends on (diopters of difference)
Two-line focus
Placeholder: each meridian focuses light at a different depth.
Blurred Image at All Distances
Because no plane holds a sharp point focus, images blur near, far, and in between.
- Blurred: Near vision (lines still separated)
- Blurred: Far vision (lines still separated)
- Least confusion: Best plane (still not sharp)
- Eye strain: Symptom (common complaint)
Everyday effect
Placeholder: streaking or doubling appears on fine detail.
Astigmatism Axis and Magnitude
Correction requires both a diopter magnitude and an axis in degrees.
- 0–180°: Axis range (standard notation)
- Diopters: Magnitude unit (cylinder power)
- Toric lens: Correction tools (glasses, contacts, LASIK)
- -2.00 x90: Notation example (cyl power at axis)
Reading a prescription
Placeholder: cylinder value and axis together define the fix.
Placeholder: toric correction flattens the steep meridian only.