Resolution of $90\mathrm{nm}$ was achieved with a research microscope simply by replacing the standard bright-field condenser with a homebuilt illumination system with a cardioid annular condenser. Diffraction gratings with $100\mathrm{nm}$ width lines as well as less than $100\mathrm{nm}$ size features of different-shaped objects were clearly visible on a calibrated microscope test slide. The resolution increase results from a known narrower diffraction pattern in coherent illumination for the annular aperture compared with the circular aperture. This explanation is supported by an excellent accord of calculated and measured diffraction patterns for a $50\mathrm{nm}$ radius disk.