Method and device for determining the optimal position of the focal plane for examining a specimen by microscopy

Determining the optimal position of the focal plane for examining a specimen 2 by microscopy by: a illuminating the specimen with light and recording images B1-B7 of the illuminated specimen at different positions zB1, zB2, ... zB7 of the focal plane such that a stack of intensity images B1-B7 of the illuminated specimen is available, b calculating a phase image P1, P2, ... P5 from at least two intensity images B1-B7, with the calculated phase image P1-P5 being assigned a focal plane position zP1, zP2, ... zP5 located within a focal plane region whose boundaries are the two most spaced apart positions zB1-zB5 of the focal plane of the at least two intensity images B1-B7, c repeating step b multiple times with different intensity images B1-B7 such that a stack of phase images P1-P5 is available, d calculating at least one focus measure value for each phase image P1-P5, and e determining the optimal position zopt of the focal plane on the basis of the calculated focus measure values and the focal plane positions zP1-zP5 assigned to the phase images P1-P5.