We present estimates of the Fresnel zone diameter for the axial magma chamber lens along the southern and northern East Pacific Rise and compare them to the magma lens widths derived from migrated seismic sections. The mean values of that diameter are 996m and 1052m and the mean values of the magma lens width are 680m and 1056m respectively. Most individual estimates of the magma lens width fall below the Fresnel zone diameter. Similar results are presented for three other mid-ocean ridges. The registered signal is therefore a mix of responses from the magma sill and the encasing rock and lateral resolution is not achieved. We conclude that data analysis procedures that assume a laterally homogeneous Earth, cannot provide reliable estimates of the magma lens physical properties, including whether it is liquid or not. ¿ 1999 American Geophysical Union