What is the approximate central corneal area measured by a keratometer in millimeters?

• A. 2 mm
• B. 3 mm
• C. 4 mm
• D. 5 mm

Answer: (B)

Keratometry reads the curvature of the central cornea by analyzing the reflection of mires in a small, central area. That measurement zone is about 3 millimeters in diameter. This size is ideal because it targets the most uniform part of the cornea, giving a stable, repeatable reading that’s useful for refractive calculations and contact lens fittings. Using a larger area would bring in peripheral curvature, which varies more and would skew the results, while a much smaller area could be harder to measure reliably. So, the central corneal area examined by a keratometer is approximately 3 millimeters across.