Effective spring constant as a function of sample stiffness. (A) Force versus z piezo displacement curve in case of sample spring constant larger, the same or lower compared to the spring constant of the cantilever. (B) Effective spring constant (k
eff, representing the slope of the force curves in A) as a function of the stiffness of the sample. From the slope of this curve it is clear that the maximum sensitivity is achieved when both spring constants are of the same order of magnitude.