Which statement correctly describes the meaning of the difference between Ks readings with respect to astigmatism?

• A. The difference is not related to astigmatism
• B. The difference indicates thickness
• C. The difference indicates the amount of astigmatism
• D. The difference indicates axial length

Answer: (C)

The key idea is that keratometry measures the cornea’s curvature in two principal meridians, and the difference in dioptric power between those meridians tells us how much astigmatism the cornea has. A cornea that isn’t perfectly spherical has different curvatures in different directions, so the flat meridian and the steep meridian will have different keratometric powers. The magnitude of this difference directly corresponds to the amount of keratometric astigmatism. For example, if the flat meridian is 43.00 D and the steep meridian is 44.50 D, the difference is 1.50 D, indicating 1.50 D of corneal astigmatism. This measure reflects curvature, not thickness (which pachymetry would assess) or axial length (which relates to overall eye size), and it doesn’t alone describe the full refractive error created by the lens or posterior cornea.