Abstract: A machine for machining a lens blank includes first and second lens blank holders which may be moved alternately between a loading station, where the lens blank can be loaded in the holder, and a machining station where the lens blank in the holder is to be machined. The machining of a lens blank at the machining station is controlled by a signal processor in response to signals from a lens blank sensing apparatus at the loading station. The sensing apparatus comprises means for sensing the position of a peripheral edge of the lens blank, and a plurality of probes engageable with different surface portions of the lens blank, means being provided for sensing the displacement of the probes and feeding to the signal processor output signals indicative of the displacement of the probes and hence indicative of the surface shape of the lens blank.