Lang, M. C., Staudt, T., Engelhardt, J. and Hell, S. W.
4Pi microscopy with negligible sidelobes. New Journal of Physics 10: 043041-13(2008).

The coherent addition of the wavefronts of two opposing high-angle lenses provides an axial (z) resolution improvement by 5-7-fold in far-field fluorescence microscopy. However, all microscopy concepts based on this principle have so far required mathematical deconvolution of the acquired data. This stems from the fact that the decrease of the axial width of the effective point spread function (EPSF) is accompanied by a substantial elevation of the side maxima of the EPSF along the optical axis. Here, we realize an EPSF with negligible lobes and gain axially superresolved images just through the physical phenomena involved. The constructive interference of the added wavefronts can be controlled through the image brightness which greatly simplifies the operation of the system.