This technology includes an illuminator and reflector that enables flexible standing wave illumination on an inverted microscope stand, and procedures for using such illumination to improve axial resolution in confocal or instant SIM imaging systems. The axial resolution in conventional fluorescence microscopy is typically limited by diffraction to ~700 nm. This method that improves axial resolution ~7-fold over the diffraction limit, and that can be applied to any fluorescence microscope. By exciting the sample with standing wave illumination of different spatial frequencies, acquiring a corresponding set of images, and processing them appropriately, the axial resolution of any widefield microscope may be improved. When using a photoswitchable fluorophore, the same method can be applied to improve the axial resolution of any microscope provided that the standing waves photoactivate the sample appropriately.