The limits of diffraction limits In your recent article on high-resolution microscopy (July/August 2011, p. 31), the heading asks, “Can optical microscopy further shred the Abbe diffraction limit?” Although the statement about shredding the Abbe diffraction limit often is perpetuated by those working in the field of STED [stimulated emission depletion] microscopy, it is basically wrong. The Abbe resolution criterion (or Rayleigh, for that matter) is valid only for real imaging systems like a microscope lens. It basically teaches us that, even if the best optical materials are used in the lens manufacturing, the resolution of the lens will be limited by diffraction of the lens aperture. The new, and impressive, scanning techniques like STED and PALM [photoactivated localization microscopy] rely on a clever interplay between fluorophores and sophisticated laser scanning methods and, as such, are more related to raster scanning known in scanning electron microscopes. It is my hope that the erroneous comparison of classical imaging and laser scanning methods will be avoided in the future. Lars René Lindvold Senior Scientist, Radiation Research Div. Risø National Laboratory for Sustainable Energy Technical University of Denmark