Centre-of-mass motion and exchange effects can be considered in nuclear knock-out by real and/or virtual photons by means of effective charges in order to retain the attractive freature of single-particle transitions. These effective charges are here obtained by splitting the energy-weighted sum rule for the charge operator into a proton and a neutron contribution. Theq-dependence of such effective charges is investigated for several nuclei. Asq→0 the dipole limit is recovered, while forq→∞ one obtains the quasi-free scattering limit. A simple model is used for the exclusive reaction both with real and virtual photons to define the corresponding effective charges which are more sensitive to the details of the reaction kinematics. The ratioep2/ep2 is in agreement with the (γ, p)/(γ, n) data.