A method of determining a focused output signal of a sensor array comprising a plurality of sensors, each sensor being operable to output a sensor output signal responsive to a measured quantity, the focused output signal being indicative of a calculated quantity at a focus point; the method comprising: receiving a respective measured sensor output signal from each of the sensors; computing the focused output signal by performing a focusing calculation with respect to the measured sensor signals; wherein the method further comprises determining a subset of mesh points of a set of predetermined mesh points, each mesh point having at least one pre-computed filter parameter associated with it; and wherein computing the focused output signal comprises performing an interpolation with respect to the subset of mesh points so as to obtain an interpolated focused output signal.