A hard-copy-unit (HCU) or camera for recording medical images on film needs regular calibration to compensate for differences in film and film processing. This can be done by making a hard copy (HC) with a number of predetermined grey values and measuring the resulting optical densities on the hard copy. From the resulting optical density values new brightness (B) and contrast (C) parameters are determined that will result in the correct maximum and minimum optical densities of subsequently exposed and processed images on film.