The invention relates to a method in the control of the quality or the condition of a fibre web (21) on the basis of optical imaging diagnostics, wherein the fibre web (21) under examination and/or means (22, 23) relating to the processing of the fibre web are monitored with at least one optical imaging measuring device (1 to N). According to the method, the scale of the observation area of said at least one imaging measuring device (1 to N) is calibrated, when said measuring device is taken into use, by means of one or more calibration objects placed in the observation area of the measuring device, to correct the perspective error caused by the position between said measuring device and the object (21, 22, 23) observed by it.
