A calibration error correction device for an optical instrument includes a detector operable to detect a position of a focusing lens of an optical instrument along a mechanical path of the focusing lens. A line of sight through an image plane of the optical instrument and the focusing lens at a present position defines an actual viewing direction. The device also includes a memory configured to store viewing direction errors specifying a deviation between a known theoretical viewing direction and the actual viewing direction associated with a plurality of different positions of the focusing lens along the mechanical path and an indicator of at least one value indicative of the actual viewing direction based on the theoretical viewing direction and the viewing direction errors at each of the different positions of the focusing lens along the mechanical path.