  The H.E.S.S. cameras require a precise and regular calibration over time, to
reconstruct the gamma-ray characteristics. The different sub-systems used to
determine the gain and the uniformity of the PMTs and their evolution with time
are presented. Then, we focus on the absolute energy scale calibration, by
using a full reconstruction of isolated muons recorded during normal
observation. The method and the evolution of the absolute overall light
collection efficiency are shown.
