We study the ac-response of a vortex lattice in type-II superconductors under strong pinning conditions and provide quantitative results for the ac penetration depth ?C, the so-called Campbell length. Contrary to the common belief that ?C is related to the curvature (second derivative) of the pinning potential at its minimum, we show that the Campbell length relates to the slope (first derivative) of the potential, evaluated at a position that depends on the preparation of the state, e.g., field cooled or zero-field cooled (Bean state). Consequently the Campbell length differs (in some cases even parametrically) for different modes of sample preparation. We find that under specific conditions ?C exhibits hysteretic behavior upon thermal cycling.