Deflection unit (10, 110, 210) comprising: - a first window, through which an optical work beam can enter the deflection unit, - a second window, through which the work beam can exit from the deflection unit and through which detection beams can enter the deflection unit from a work region along the corresponding trajectory of the outgoing work beam and in a direction opposite thereto, - an optical element reflecting a first wavelength at least partially, wherein the deflection unit - defines a work beam path followed by the work beam from the first window to the second window via a reflection at the optical element, and characterised in that the deflection unit further comprises: - an XY deflection device arranged in the work beam path between the second window and the optical element for deflecting the work beam and for scanning the outgoing work beam, and - a focusing device for focussing the work beam, wherein the focusing device comprises at least two movable lenses (54, 56) and has a variable focal length and is arranged in the work beam path between the first window and the optical element, wherein the focusing device is configured for moving at least two lenses of the at least two movable lenses (54, 56) simultaneously and independently of each other and for modifying a focus diameter of the work beam without modifying at the same time a focus position; wherein the deflection unit (10, 110, 210) further comprises a control unit configured for controlling shiftings of the at least two movable lenses (54, 56) of the focusing device along the working beam path and for adjusting the focal length of the focusing device as a whole depending on an adjustment of the XY deflection device. - a first window, through which an optical work beam can enter the deflection unit, - a second window, through which the work beam can exit from the deflection unit and through which detection beams can enter the deflection unit from a work region along the corresponding trajectory of the outgoing work beam and in a direction opposite thereto, - an optical element reflecting a first wavelength at least partially, wherein the deflection unit - defines a work beam path followed by the work beam from the first window to the second window via a reflection at the optical element, and - defines a work beam path followed by the work beam from the first window to the second window via a reflection at the optical element, and characterised in that the deflection unit further comprises: - an XY deflection device arranged in the work beam path between the second window and the optical element for deflecting the work beam and for scanning the outgoing work beam, and - a focusing device for focussing the work beam, wherein the focusing device comprises at least two movable lenses (54, 56) and has a variable focal length and is arranged in the work beam path between the first window and the optical element, wherein the focusing device is configured for moving at least two lenses of the at least two movable lenses (54, 56) simultaneously and independently of each other and for modifying a focus diameter of the work beam without modifying at the same time a focus position; - an XY deflection device arranged in the work beam path between the second window and the optical element for deflecting the work beam and for scanning the outgoing work beam, and - a focusing device for focussing the work beam, wherein the focusing device comprises at least two movable lenses (54, 56) and has a variable focal length and is arranged in the work beam path between the first window and the optical element, wherein the focusing device is configured for moving at least two lenses of the at least two movable lenses (54, 56) simultaneously and independently of each other and for modifying a focus diameter of the work beam without modifying at the same time a focus position; wherein the deflection unit (10, 110, 210) further comprises a control unit configured for controlling shiftings of the at least two movable lenses (54, 56) of the focusing device along the working beam path and for adjusting the focal length of the focusing device as a whole depending on an adjustment of the XY deflection device.