We have studied the effect of lateral microstructuring on the upper critical field of superconducting Pb films with a square antidot lattice of period d, formed by square antidote with size a x a. The critical field was determined resistively by keeping the resistance at a fixed value while varying field and temperature. In low fields, the critical field curve demonstrates a cusp-like structure with cusps at integer and rational multiples of Phi(o)/d(2). At low fields, the periodicity is thus governed by the size of the lattice cell and corresponds to a superconducting wire network regime. In higher fields, the crossover from a ''collective'' to a ''single object'' behaviour is observed, characterized by oscillations with a period Phi(o)/a(2) coming from the formation of superconducting edge states around the antidots.