Ophthalmology

A 35-year-old woman presents with blurred vision and eye strain. Refraction reveals +2.00 D sphere with +1.50 D cylinder axis 90°. On retinoscopy, the red reflex moves in opposite directions along the two principal meridians. Which optical principle explains this finding?

A 28-year-old patient with -4.00 D myopic astigmatism (-3.00 D sphere -1.50 D cylinder × 180°) is being evaluated for contact lens fitting. The horizontal corneal curvature is 44.00 D and vertical is 45.50 D. Which contact lens prescription would be most appropriate for correcting both the myopia and astigmatism?

A 55-year-old man with corneal astigmatism of +1.50 D × 180° (from keratoconus) undergoes corneal cross-linking (CXL). Six months later, his astigmatism has increased to +2.25 D × 180°. Anterior segment OCT shows stable corneal thickness and no evidence of progression. Which factor most likely accounts for the increased astigmatism despite stabilization?