Photographic films are used as detectors in x-ray microscopes. A criterion for the sensitivity of film emulsions to soft x-ray radiation is the detective quantum efficiency (DQE). To keep the radiation dose transfered to the observed object as small as possible, one is interested in emulsions with a high DQE. Measurements have shown, that Ilford nuclear research emulsions have a much higher DQE than conventional films. A methode to determine the DQE of film emulsions and the results of the measurements will be described in detail.