A surgical instrument system comprises an instrument and an optical tracking system (1), the instrument comprising a first part and a movable second part (6) which is movable relative to the first part. A light source (4) is disposed on the first part. The movable second part (6) comprises a reflective surface (5), wherein the movable second part (6) is movable in the direction of the axis of a directional light beam emitted from the light source (4), wherein the position of the movable second part (6) with respect to the first part is measurable by means of the optical tracking system (1) by means of the light beam reflected on the reflective surface (5) of the movable second part (6), wherein the first part can comprise at least three light emitters in a known geometric arrangement, which can form an optical marker (2).
