Fits of pion--nucleus potentials to large sets of pionic atom data reveal departures of parameter values from the corresponding free \pi N parameters. These medium effects can be quantitatively reproduced by a chiral-motivated model where the pion decay constant is modified in the medium or by including the empirical on-shell energy dependence of the amplitudes. No consistency is obtained between pionic atoms and the free \pi N interaction when an extreme off-shell chiral model is used. The role of the size of data sets is briefly discussed.

PACS numbers: 12.39.Fe, 13.75.Gx, 21.30.Fe, 36.10.Gv