Since in-depth sensing indentation load-depth data of the entire loading-unloading cycle are available, more information than a single hardness value and an elastic modulus can be extracted from the experimental data. The conventional hardness H (h)= F (h)/A C (h) and the differential hardness H d (h)= dF/dA C are calculated as continuous functions of depth h and compared to each other in this paper (F: load, A C: contact area). It turns out that H d describes the momentary material resistance to deformation, whereas H integrates over deformation states from first tip-sample contact to current penetration h. This difference is particularly important for materials not homogeneous in depth (eg layer systems), and for situations where time-dependent external factors influence the momentary deformation resistance. Photoplasticity is considered as an example for the latter.