In certain embodiments, an ophthalmic system for determining the topography of the anterior surface of the cornea of an eye comprises an illuminator, a camera, and a computer. The illuminator illuminates the anterior surface of the cornea of the eye with a Placido pattern. The Placido pattern comprises a plurality of rings. A ring of the plurality of rings has a distinguishing feature that distinguishes the ring from an adjacent ring. The anterior surface of the cornea reflects the Placido pattern. The camera captures an image of the reflected Placido pattern. The computer: analyzes the image to detect a distortion of the ring that indicates an anomaly of the anterior surface of the cornea; identifies the ring of the plurality of rings according to the distinguishing feature of the ring; and generates the topography of the surface of the cornea that includes the anomaly.