Geisler, C., Schonle, A., von Middendorff, C., Bock, H., Eggeling, C., Egner, A. and Hell, S. W.
Resolution of λ/10 in fluorescence microscopy using fast single molecule photo-switching. Applied Physics A 88: 223-226 (2007).

We demonstrate nanoscale resolution in far-field optical microscopy based on photo-switching of molecules. By enabling, recording and disabling fluorescence from individual labels sequentially, the detection volume is reduced to the size of a single molecule and the diffraction limit is broken. Images of nanostructures milled into a coverslip and tagged by fluorescent proteins could be recorded at 50 nm resolution. Due to the fast and asynchronous image acquisition protocol used in these experiments, we were able to reduce acquisition times to ~ 2.5min, which is two orders of magnitude lower than in previous implementations.