A fast method of calculating the diffraction loss between two facing circular ultrasonic transducers of unequal size is presented. This problem is directly applicable for minimization of diffraction loss in acoustic lens design. Graphs for amplitude and phase are presented that can be used to design lenses with the optimal transducer size for minimum diffraction loss. The theory is extended to include the diffraction loss determination in anisotropic materials. The results are in good agreement with previous experimental and theoretical results of the equal transducer size case. The effect of diffraction on pulsed excitation is also treated.