...
The resolution of light microscopy is traditionally limited by diffraction to approximately half the wavelength of light, which means any objects smaller than about 250 nm cannot be resolved from one other using light. In recent years super-resolution microscopy techniques have been developed to extend light microscopy beyond this diffraction limit. These include Image Scanning Microscopy (ISM), Structured Illumination Microscopy (SIM), Stimulated Emission Depletion microscopy (STED) and Single-molecule Localisation Microscopy (SMLM).
More about super-resolution microscopy techniques...
...