The focusing operation of a microscope, control rotary knobs which receive the motion of fingers or hand of an operator to change the focus position relative to a sample under observation. Two of the control rotary knobs are on a common shaft and coaxial with one another and rotationally fixed with respect to one another, and are connected to a position encoder. A toggle switch which is connected to the control circuit and can be actuated by touching or moving one or both rotary control knobs. Whereby the drive circuit is configured such that it generates, as a function of the rotation of the two rotary knobs and as a function of the actuation or position of the toggle switch, a rough or fine adjustment of the focus position relative to the sample under observation.