stepping motor sm is arranged so as to be fixed with respect to rotation and freely displaceable relative to plate p 1 it synchronously drives the spindles sp 1 and sp 2 having threads with different pitch e g threads m3x0 5 and m2 6x0 45 result in a pitch difference of 50 u03bcm spindle sp 2 is driven by stepping motor sm so as to screw into the nut m 2 fastened to the frame f 2 whereupon the stepping motor sm which is fixed with respect to rotation is displaced along axis a by the spindle displacement in the direction indicated by the arrows nut m 1 which is fastened to the guide part f 1 is displaced relative to nut m 2 due to the differential thread by an amount approximately 10 times less than that of the stepping motor sm this causes a very high sensitivity in the adjustment of f 1 the diagonal spring webs st 5 st 6 which spread apart or close when the motor shaft rotates are fastened to guide part f 1 which is connected by spring webs parallel spring joint to the frame and open and close the plates p 1 p 2 and therefore the pinhole while the frame.