The present invention relates to a computer-implemented medical method ofdetermining a spatial position of a medical optical observation device (1),the methodcomprising executing, on a processor of a computer (2), the steps of:- acquiring position data describing, for a plurality of points in time,the spatialposition of the observation device (1) within a co-ordinate system of amedicaltracking system (3)；- determining, based on the position data, average position data describinganaverage value for the position of the observation device (1) within the co-ordinatesystem of the medical tracking system (2)；- acquiring image data describing a plurality of images acquired at theplurality ofpoints in time via a camera (4) assigned to the observation device (1) anddetectingthe field of view (5) of the observation device (1)；- determining, based on the image data, optical flow data describing anoptical flowfor the plurality of images；- determining, based on the average position data and the optical flowdata, focalplane position data describing a spatial position of the focal plane (6) oftheobservation device (1).The present invention further relates to a corresponding computer-program, acorresponding computer storage medium and a corresponding system fordetermining the spatial position of a medical optical observation device.
