A microscope device comprising: an objective ; an actuator for adjustment of a distance between the objective and a sampleholding substrate in a focusing direction z during a focusing action to reach and hold a desired focus position; an absolute z-position detector for measuring a z-position of the actuator in the focusing direction; an autofocus light source ; a first optical arrangement for generating a focused spot of autofocus light from the light source in the backfocal plane of the objective at a position offset from the optical axis of the objective by a lateral offset distance ( d ), such that the objective generates a collimated incoming beam of autofocus light, which is directed at an oblique angle (β) relative to the optical axis of the objective onto the substrate; a second optical arrangement for generating collimated outgoing beams of autofocus light from the collimated incoming beam as reflected by a substrate surface through the objective and directing the collimated outgoing beam onto a sensor-array comprising a plurality of sensor elements ; a relative z-position detector comprising the sensor-array configured to detect the wandering position of the outgoing collimated autofocus light beam on the sensor-array during the focusing action, the wandering position reflecting a change in the distance between the objective and the reflecting substrate surface in the z-direction; and a central control unit configured to control the actuator based on signals from the absolute z-position detector and the relative z-position detector, wherein the signal of the relative z-position detector is used to dynamically recalibrate the absolute z-position detector during the focusing action. an objective ; an actuator for adjustment of a distance between the objective and a sampleholding substrate in a focusing direction z during a focusing action to reach and hold a desired focus position; an absolute z-position detector for measuring a z-position of the actuator in the focusing direction; an autofocus light source ; a first optical arrangement for generating a focused spot of autofocus light from the light source in the backfocal plane of the objective at a position offset from the optical axis of the objective by a lateral offset distance ( d ), such that the objective generates a collimated incoming beam of autofocus light, which is directed at an oblique angle (β) relative to the optical axis of the objective onto the substrate; a second optical arrangement for generating collimated outgoing beams of autofocus light from the collimated incoming beam as reflected by a substrate surface through the objective and directing the collimated outgoing beam onto a sensor-array comprising a plurality of sensor elements ; a relative z-position detector comprising the sensor-array configured to detect the wandering position of the outgoing collimated autofocus light beam on the sensor-array during the focusing action, the wandering position reflecting a change in the distance between the objective and the reflecting substrate surface in the z-direction; and a central control unit configured to control the actuator based on signals from the absolute z-position detector and the relative z-position detector, wherein the signal of the relative z-position detector is used to dynamically recalibrate the absolute z-position detector during the focusing action. <EOS>