The present invention relates to a computer-implemented medical method of determining a spatial position of a medical optical observation device (1), the method comprising executing, on a processor of a computer (2), the steps of:—；acquiring position data describing, for a plurality of points in time, the spatial position of the observation device (1) within a co-ordinate system of a medical tracking system (3)；—；determining, based on the position data, average position data describing an average value for the position of the observation device (1) within the co-ordinate system of the medical tracking system (2)；—；acquiring image data describing a plurality of images acquired at the plurality of points in time via a camera (4) assigned to the observation device (1) and detecting the field of view (5) of the observation device (1)；—；determining, based on the image data, optical flow data describing an optical flow for the plurality of images；—；determining, based on the average position data and the optical flow data, focal plane position data describing a spatial position of the focal plane (6) of the observation device (1). The present invention further relates to a corresponding computer-program, a corresponding computer storage medium and a corresponding system for determining the spatial position of a medical optical observation device.
