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A keratometer is a diagnostic instrument used for measuring the curvature of the anterior surface of the cornea, particularly for assessing the extent and axis of astigmatism. It measures the central 3 mm of the cornea, which generally behaves like a sphere or a spherocylinder. A keratometer in relation to the cornea is just like an object in front of a convex reflecting mirror. Like in a convex reflecting surface, the image is located posterior to the cornea. The cornea behaves as a convex reflecting mirror and the mires of the keratometer are the objects. The radius of curvature of the cornea's anterior surface determines the size of the image.