One of the well-known methods of ultrasound focusing is the use of a transducer having the form of a concave spherical cap. In the theoretical analysis, O’Neil’s formula is widely used, which describes acoustic pressure on the axis of the transducer under the assumption that normal velocity of its surface is uniform. Usually, it is believed that modern piezoelectric focused sources are very close to this theoretical model. The paper presented describes results of experimental investigation which showed that there exists a significant disagreement between the observed acoustic field and that predicted by the theory of O’Neil. Two types of measurements have been made to clarify this problem. The acoustic pressure field of a focused piezoceramic transducer (with aperture diameter of 100 mm, radius of curvature of 100 mm, resonance frequency of 1 MHz) has been measured in a water tank using a miniature hydrophone. In addition, the radiator surface vibration has been studied in air using an optical interferometric technique. Theoretical analysis and the measurements show that the disagreement between the theory and the experiment is due to Lamb waves, which originate at the radiator edge, propagate along the piezoceramic plate without significant attenuation, and thus radiate additional acoustic perturbations.