Data collected by the NA57 experiment at the SPS in Pb-Pb collisions at 40 GeV/c beam momentum per nucleon are analyzed over a centrality range corresponding to the most
central 53% of the Pb-Pb inelastic cross-section. Hanbury-Brown and Twiss correlation functions of negative hadrons (mainly pions) are studied as a function of transverse momentum and rapidity. The results are analyzed based on the blast-wave model and the source parameters - freeze-out temperature, transverse flow velocity, duration
of the expansion and duration of pion emission - are extracted as a function of centrality. The results are compared with those of other experiments and with the blast-wave analyses of the single particle p_t spectra of strange particles (K0s, lambda, antiLambda, Xi, antiXi, Omega). Compatible results are found, thus suggesting similar thermal freeze-out conditions for pions and strange particles.