Microscope with retain focus control

A microscope, comprising:

• imaging optics;
• a stage that supports a sample to be examined during an examination interval;
• a motion unit operably coupled to either the stage or the imaging optics that adjusts a distance between the stage and the imaging optics;
• a focus measurement unit that measures an actual focal position and emits a focus measurement signal;
• a control unit operably coupled to the motion unit and the focus measurement unit that maintains a predetermined focal position for temporally spaced examinations of the sample;
• wherein the control unit receives the focus measurement signal and derives therefrom a derived deviation between the actual focal position and a predetermined focal position, and depending on the derived deviation, changes the distance between the stage and the imaging optics by operation of the motion unit such that that the predetermined focal position is substantially maintained;
• further wherein the control unit triggers the motion unit to maintain the predetermined focal position only before and/or after at least one of the examination intervals but never during the examination interval;
• further wherein the control unit continuously determines the deviation between the predetermined focal position and the actual focal position while the motion unit is at rest and is not adjusting the distance between the stage and the imaging optics, and wherein the control unit effects substantial maintenance of the predetermined focal position by causing the motion unit to adjust a distance between the stage and the imaging optics when the deviation between the predetermined and the actual focal position exceeds a threshold value;
• and
• further wherein the threshold value is adjusted automatically dependent upon the imaging optics.