A method and system for size calibration of an electronically generated image of a specimen that is generated by an optical instrument having a downstream digital camera operable in different reproduction modes is disclosed. In order to enhance image analysis reliability and decrease access times, it is proposed that firstly, for a specified reproduction mode of the camera, a reference calibration value indicating the ratio of a specimen dimension to an image dimension be determined and stored together with the specified reproduction mode; and that for size calibration, a correction factor regarding the reproduction mode be derived by comparing the stored reproduction mode to the reproduction mode of the camera currently in use, and from that correction factor, together with the stored reference calibration value, the current calibration value be calculated.