A microscope includes an illumination unit for illuminating a mask at a predetermined non-axial illumination angle, an imaging unit for imaging an aerial image of the mask within a predetermined defocus region, and an imaging field stop, in which as a result of the lateral displacement of the aerial image depending on the position within the defocus region and on the non-axial illumination angle, the opening of the imaging field stop is dimensioned such that the aerial image is either completely encompassed or circumferentially cut within the defocus region.A method for characterizing a mask having a structure includes illuminating the mask at at least one illumination angle using monochromatic illumination radiation such that a diffraction image of the structure is created, recording the diffraction image, establishing the intensities of the maxima of the adjacent orders of diffraction, and establishing an intensity ratio of the intensities.