A method for estimating a stiffness of a deformable part of a system including a four-photodiode detector for analyzing at least one characteristic of a sample. The method includes receiving the signals recorded by the four photodiodes, calculating the resultant signals from the recorded signals, calculating a cross-correlation of the resultant signals calculated for obtaining an intercorrelated signal, estimating the stiffness of the deformable part depending on the intercorrelated signal.