The free-field sensitivity of a microphone is related to the product of its pressure sensitivity, the scattering factor and the load of the radiation impedance on the diaphragm. The product of the last two factors describes how much the sensitivity of the microphone differs from its pressure sensitivity. The modulus of the ratio of the free-field sensitivity to the pressure sensitivity is called the free-field correction when expressed logarithmically. By definition, the free-field sensitivity implies that a plane wave is impinging on the microphone. However, in practice the free-field reciprocity technique is based on the substitution of the microphones by point sources with a given diffraction factor. At sufficiently long distances spherical waves approximate plane waves, but this may not be the case when the microphones are calibrated using the reciprocity technique where the microphones are usually quite close to each other. The problem has been analysed numerically and experimentally. Experimental values of the free-field correction are found to be in agreement with standardized values.