A method of determining a focal length of a camera and/or of adjusting a viewing direction in a graphical representation of a pre-operative image is provided. The method includes providing uncalibrated camera data of a camera, specifying an initial value of a focal length of the camera, specifying a working distance value of a distance between the camera and the at least part of the tracking system, calculating a distance value of the distance between the camera and the at least part of the tracking system based on the uncalibrated camera data and based on the specified initial value of the focal length of the camera, calculating a change factor based on the specified working distance value and the calculated distance value, and calculating an adapted value of the focal length of the camera based on the initial value of the focal length and based on the change factor.