Method of calibrating a stereomicroscope and a stereomicroscope capable of being calibrated

A method of calibrating the magnification of a stereomicroscope, the stereomicroscope comprising an optical zoom system (4) for continuously varying the magnification of the stereomicroscope, the zoom system including at least one fixed optical lens member (4a, 4d) and at least one movable optical lens member (4b, 4c) aligned along an optical axis (2) of the zoom system, wherein the at least one movable optical lens member is axially positionable relative to the at least one fixed optical lens member along the optical axis of the zoom system through a continuous range of positions to vary the magnification, the zoom system having a zoom position defined by an axial position of the at least one movable optical lens member, the method comprising the steps of:

• positioning an object reference measure (10) in an object plane (8) of the stereomicroscope, the object reference measure having scale markings of known spacing;
• providing an eyepiece reference measure (6a) in an intermediate image plane (7) of the stereomicroscope, the eyepiece reference measure having scale markings of known spacing;
• varying the magnification of the stereomicroscope using the zoom system (4) so that an image of the object reference measure (10) is brought into optical coincidence with the eyepiece reference measure (6a) in the intermediate image plane (7), such that at least two scale markings of the object reference measure are optically aligned in the intermediate image plane with at least two scale markings of the eyepiece reference measure;
• determining an actual magnification of the stereomicroscope in the zoom position corresponding to the optical coincidence on the basis of a lateral magnification defined by the aligned scale markings of the of the object reference measure (10) and the eyepiece reference measure (6a);
• determining a nominal magnification of the stereomicroscope in the zoom position corresponding to the optical coincidence;
• correlating the actual magnification and the nominal magnification by computer to calculate a correction factor at the zoom position;
• and
• calculating magnifications for other zoom positions of the zoom system using the correction factor and corresponding nominal magnifications respectively associated with the other zoom positions.