Microscope, in particular for the measurement of total internal reflection fluorescence, and operating method for such

relatively weak light sources have been used with little light intensities in wide field microscopes. The focusing in the pupil plane leads inevitably to low light intensities in the sample, since the power of the light source is distributed to a very large sample area. However, there are wide-field techniques that require very high intensities, for example, PAL microscopy. The invention is to enable the flexible adjustability of the illumination light intensity in the sample with little effort. For this purpose, a laser is used and disposed a variable lens in the illumination beam path, which enables a variable adjustment of a beam cross section of the illumination light in an intermediate image plane as a light source, wherein a divergence of the illumination light for different beam cross-sections are identical. Thus, the field of view of the microscope can be altered flexible in its size. Thus, the intensity of the laser illumination light can be varied in the sample in a wide range of values. fluorescence microscopy