  H.E.S.S. - the High Energy Stereoscopic System- is a new system of large
atmospheric Cherenkov telescopes for GeV/TeV astronomy. Each of the four
telescopes of 107 m^2 mirror area is equipped with a 960-pixel
photomulitiplier-tube camera. This paper describes the methods used to convert
the photomultiplier signals into the quantities needed for Cherenkov image
analysis. Two independent calibration techniques have been applied in parallel
to provide an estimation of uncertainties. Results on the long-term stability
of the H.E.S.S. cameras are also presented.
